John Woods Campbell
Ken Arromdee
>If
>you'd ever read the published volume of his letters you'd know that a lot of
>this was showmanship, intended to jar people into thinking outside their
>preconceptions.
Unfortunately, not every bad argument equally encourages thinking. For
instance, one which depends on a factual error doesn't encourage thinking,
except in the limited sense that checking a reference book is "thinking". This
is still so if the factual error is phrased as a conjecture. (One common
crackpot technique is to make scattershot claims phrased as conjectures; if the
crackpot stumbles on anything, he takes credit for it, while denying he meant
the others as anything but suggestions--after all, he didn't _say_ they were
_true_, he was just throwing out ideas.)
Similarly, bad arguments that depend on elementary logical errors don't really
encourage thinking, except again in a limited, simplistic, sense.
"It's to make people think" is a lazy person's excuse--one could _always_ make
that excuse for _any_ argument, no matter what one's actual motives are. And
it's impossible to disprove.
>For that matter, the final verdict on the Dean drive isn't in yet; the research
>that would have done so was defunded for political reasons in the early 70s and
>never resumed. There's an extension of Newtonian mechanics that adds a `surge'
>term reflecting the fact that objects don't respond instantaneously to applied
>force --- the pressure wave needs time to propagate from point of application
>across the object, and this can interact with velocity and elasticity in some
>ways that conventional theory does not predict. This extension (the "Davis
>Mechanics" of Heinlein's _Podkayne_Of_Mars_, named after the developer, Greg
>O. Davis) actually predicts and models the Dean Drive.
But the statement you just made about objects isn't true. Action and reaction
happen simultaneously; or rather, action and reaction happen at any point
simultaneously. In some sense there's a pressure wave that has to propagate,
but if you take a snapshot of something during a pressure wave you'll find that
the total momentum of the object stays the same even though the wave didn't
reach the end of the object yet. Interactions with velocity and elasticity do
not change this momentum.
Why is it that Campbell defenders seem to use the "I didn't steal your pot,
and anyway I returned it in perfect condition" argument? If you want to argue
that Campbell didn't really support the things we criticize him for, fine.
If you want to argue that Campbell did support them, but that they're not
crackpot ideas, fine. But to argue both is contradictory; if Campbell was only
throwing out ideas and didn't really think the Dean Drive works, why are you
defending Campbell by trying to convince me it _does_ work?
>The SF mainstream he defined outlasted them; it will
>outlast the current `PC' furor; and it will almost certainly outlast *you*.
The nice thing about predictions about the future is that you have to wait
until the future to see if they come true. But it also makes such predictions
worthless in arguments that take place now. To say that Campbell's influence
will outlast *me* is not an argument; it is an incredibly arrogant assertion,
equivalent to "I am right and you are wrong", and it does not and cannot serve
as support for your real arguments.
Anyway, I don't see many Dean Drive SF stories. I do see stories where writers
use various drives which defy physics, but the competent writers recognize that
they do so, rather than claiming their space drives work in the real world.
For that matter, I don't even see proportionately as many "human culture is
always the best" stories as appeared before. The only real influence I see is
psi, and that's a special case--Campbell was far less responsible for psi in
SF than most people think; and much modern use of psi in SF is as a substitute
for magic in stories otherwise far closer to fantasy than science fiction.
--
"On the first day after Christmas my truelove served to me... Leftover Turkey!
On the second day after Christmas my truelove served to me... Turkey Casserole
that she made from Leftover Turkey.
[days 3-4 deleted] ... Flaming Turkey Wings! ...
-- Pizza Hut commercial (and M*tlu/A*gic bait)
Ken Arromdee (arro...@jyusenkyou.cs.jhu.edu, arro...@jhunix.hcf.jhu.edu)
Eric S. Raymond
> But the statement you just made about objects isn't true.
We shouldn't argue this one here.
> Why is it that Campbell defenders seem to use the "I didn't steal your pot,
> and anyway I returned it in perfect condition" argument?
Um...I'm not doing that, I don't think. Some of Campbell's crusades were
pure chain-yankers --- the Shaver mystery, for example. Some were advocacy
for "damned things" he thought ought to be taken more seriously; the Dean
Drive was one such. Some are hard to classify; I'm not sure whether he
actually believed in the Hieronymus Machine or not.
(My personal favorite Campbellian Damned Thing, BTW, was his belief that proto-
hominids *selected* themselves into humanity via adulthood-initiation rituals
that tested for the ability to override instinctive fears on verbal command.)
> Anyway, I don't see many Dean Drive SF stories. I do see stories where writers
>use various drives which defy physics, but the competent writers recognize that
> they do so, rather than claiming their space drives work in the real world.
> For that matter, I don't even see proportionately as many "human culture is
> always the best" stories as appeared before. The only real influence I see is
> psi, and that's a special case--Campbell was far less responsible for psi in
> SF than most people think; and much modern use of psi in SF is as a substitute
> for magic in stories otherwise far closer to fantasy than science fiction.
Your notion of Campbell's influence is far too narrow and ridiculously negative.
I suggest you read a good history of SF before we continue this discussion.
Among other things, Campbell was the first editor in SF to insist on plausible
science and *good writing*. He did not merely `influence' SF, he *defined*
SF as we know it today --- as a literature of plausible extrapolation centered
on human reactions to technological change and challenge.
Even Campbell's most vociferous critics are operating, more than they realize,
within the conceptual framework Campbell created. His reinvention of SF was
so sweeping, so complete, that it became invisible. The only way to really
grasp it is to read the ephemera of pre-Campbellian "scientifiction" --- not
Verne and Wells and Doc Smith, but the Captain S. P. Meeks and Ray Cummingses
and Otis Adelbert Klines.
I've done a good bit of this. Not many people persevere at it, because the
stuff, in general, is bloody awful. And *that* fact, if you think about it,
is all the `rescue' JWC's reputation needs.
--
Eric S. Raymond <e...@snark.thyrsus.com>
Jim Mann
arro...@jyusenkyou.cs.jhu.edu (Ken Arromdee) writes:
> Ironically, another thread mentions a space trilogy written by
Campbell. I've
> read it. It's no better than Ray Cummings.
But we're not talking about Campell the writer here. We're talking
about him as editor. Most of the stuff Campbell wrote under
his own name was no better than the run of the mill space
opera of the time (the stuff he wrote as Don A. Stuart was
better). However, he was the pivotal editor in SF history.
>
> The trouble with an argument like this is that it's unprovable.
You can say
> "Campbell SF was a big jump from pre-Campbell SF" all you want, and
give
> examples, and have no way to be proven or disproven, because
there's enough
> subjective judgment involved that we can _prove_ it only in extreme
cases, and
> this one is not extreme enough. (And I'd argue that that itself
shows that
> the jump, even if it exists, couldn't be _that_ big.)
Define reasonable standards of proof and anyone familiar with
the history of SF can prove it.
There are various telling pieces of evidence:
1. Science fiction before 1939 was very different, on average,
than SF after 1939, when Campbell took over at Astounding.
Most of the major writers, who were to have profound
influence on those who would come later, were writers
developed by Campbell.
Compare the major works of the forties--by Heinlein, Sturgeon,
Asimov, De Camp, Van Vogt (not a great writer, but, at
his best, a step up from the 1930s), Simak, Kuttner,
Moore, and so forth--to the major works of the 1920s and 1930s.
You'll find a more polished writing style, better
characterization, less of a "pulp, pot-boiler feel," more
logical backgrounds, and so forth.
2. Most of the major SF critics and historians agree on
Campbell's effects, including Knight, Blish, Aldiss,
and others.
3. The writers themselves agree on this point. Stories
that were commonplace in the 30s could no longer meet the
standards of the 40s.
>
> Who defined SF in its current form is another unprovable; I could
just as well
> claim Star Wars defined SF today. There are just so _many_ things
that
> influenced today's SF that you can't trace most specific
innovations back to
> single sources.
I don't think anyone is claiming that on single person or thing
was the source of today's SF. But Campbell was the person
who had the single biggest impact. Subsequent "revolutions"
were spread out over more different people. 1949-50 was a time
that was almost as important in SF as 1939, since this saw
the birth of F&SF and Galaxy, which together, in somewhat
different ways, widened the SF horizon. But the "credit" for
this falls on several different editors (like Anthony Boucher
and Horace Gold), while 1939 was all Campbell. Likewise,
the new wave of the sixties had a profound impact, but again
was attributable to a handful of major figures, not just a
single editor.
--
Jim Mann
Stratus Computer jm...@vineland.pubs.stratus.com
David Stein
>science and *good writing*. He did not merely `influence' SF, he *defined*
>SF as we know it today --- as a literature of plausible extrapolation centered
>on human reactions to technological change and challenge.
Sounds just like J. Verne's idea of science fiction.
>Even Campbell's most vociferous critics are operating, more than they realize,
>within the conceptual framework Campbell created. His reinvention of SF was
>so sweeping, so complete, that it became invisible.
A completely sweeping assertion, but the argument is invisible.
Can you show some evidence?
[on reading pre-Campbellian "scientifiction":]
>I've done a good bit of this. Not many people persevere at it, because the
>stuff, in general, is bloody awful. And *that* fact, if you think about it,
>is all the `rescue' JWC's reputation needs.
Wrong, even if you don't think about it.
First, the pulpish "scientifiction" was not the sole manifestation of SF
at that time period, nor was it the only known form of SF when Campbell
started. So its choice for a comparison has unclear implications even if
Campbell strogly influenced the field.
Second, the comparison does not prove that Campbell _did_ influence the
quality of SF decisively and positively.
Third, if only quality improved, perhaps that was to be expected when
pulp SF matured as its readers turned writers. This happens in many
fields. The improvement might have occured without Campbell, and may
have actually occured in spite of Campbell.
You do not explain how Campbell redirected the thinking of SF writers so
as to "reinvent" SF. You do not even show that without Campbell we would
not have good writers like Heinlein, Asimov, Simak, Sturgeon. Were they
writing SF only because of Campbell? Would they have a difficult time
being published elsewhere if Campbell were not an editor? Would they
write bad SF without Cambell?
And yes, I would prefer some evidence rather then just "yes yes yes".
- David
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I'm not a native speaker of English, so I'm not sure what I wrote.
Flames will be ignored unless you post them in perfect Czech.
================================ - David (the metamathician) - ===
Beth Moursund
>(My personal favorite Campbellian Damned Thing, BTW, was his belief that proto-
>hominids *selected* themselves into humanity via adulthood-initiation rituals
>that tested for the ability to override instinctive fears on verbal command.)
This brings back faint memories of a short story I read god-knows-how-many
years ago... I think it involved an anthropologist on some planet, who
ended up participating in such an initiation ritual. This probably
describes several dozen stories at least, but can anyone suggest what
story it might have been? Email to me and I'll post a summary.
- Beth Moursund
bet...@microsoft.com
Eric S. Raymond
>>Among other things, Campbell was the first editor in SF to insist on plausible
>>science and *good writing*. He did not merely `influence' SF, he *defined*
>>SF as we know it today --- as a literature of plausible extrapolation centered
>>on human reactions to technological change and challenge.
>
> Sounds just like J. Verne's idea of science fiction.
Plausible but wrong. Read Panshin's discussion of Verne in _The_World_Beyond_
The_Hill_ for an effective refutation.
>>Even Campbell's most vociferous critics are operating, more than they realize,
>>within the conceptual framework Campbell created. His reinvention of SF was
>>so sweeping, so complete, that it became invisible.
>
> A completely sweeping assertion, but the argument is invisible.
> Can you show some evidence?
Do your own homework (I cited some sources to start with in my reply to Ken
Arromdee). There is quite a bit of good scholarship on this subject, not to
mention first-person testimony from writers who were active before, during,
and after the Campbellian revolution.
> You do not explain how Campbell redirected the thinking of SF writers so
> as to "reinvent" SF.
I don't have to. They've done it themselves, at length, in print. Campbell's
influence is well documented. If you want to argue this further, repair your
ignorance first.
Ken Arromdee
physics groups when the Dean Drive was mentioned. If you're not going to
mention it, please take those two groups (and probably .sf.science too) _out_
of the header....
Henry Troup
arro...@jyusenkyou.cs.jhu.edu (Ken Arromdee) writes:
|>But the statement you just made about objects isn't true. Action and
reaction
|>happen simultaneously; or rather, action and reaction happen at any point
|>simultaneously.
This is one I can't figure out. Consider a steel bar, in free space, a
long way from anywhere, one light-year long. place a large rocket
against one end, and turn it on. Let it run for 364 days. Turn it off.
What has happened? Try it again with an incompressible rod - if the rod
moves, the message reached the far end faster than light. If the rod
doesn't move, where'd the momentum go?
Now shrink the system to one light-second long, and run the engine in
half second bursts. All the same effects have to be happening.
Now shrink the system to one meter and run the "engine" at 600 MHz -
again, the same 'delay' effects are there.
But Davis didn't really account for the Dean Drive - partly because the
device Dean demo'd wasn't the one in the patent drawings, and he
wouldn't let anyone closely examine his model. Dean was a classic
crackpot, as Harry Stine admits. Stine did a followup article on Davis
mechanics and some actual experiments that did have unexpected results.
Like a simple mechanical system with an unexplained 3 degree phase shift.
Henry Troup - H.T...@BNR.CA (Canada) - BNR owns but does not share my opinions
Legislated morality is the enemy of true morality
Ken Arromdee
>|>But the statement you just made about objects isn't true. Action and
>reaction
>|>happen simultaneously; or rather, action and reaction happen at any point
>|>simultaneously.
>This is one I can't figure out. Consider a steel bar, in free space, a
>long way from anywhere, one light-year long. place a large rocket
>against one end, and turn it on. Let it run for 364 days. Turn it off.
>What has happened? Try it again with an incompressible rod - if the rod
>moves, the message reached the far end faster than light. If the rod
>doesn't move, where'd the momentum go?
You can't make an incompressible rod, precisely for this reason.
Thought experiments allow us to assume things that get around real-world
engineering limitations, but not to make assumptions which are equivalent to
violations of physics; in this case, an incompressible rod is one.
So the rocket-powered end is moving and the other isn't. The local motion of
the matter that is at any moment located in the propagating wave is associated
with momentum, and this momentum remains the same as the wave progresses.
Gerry Roston
This is one I can't figure out. Consider a steel bar, in free space, a
long way from anywhere, one light-year long. place a large rocket
against one end, and turn it on. Let it run for 364 days. Turn it off.
What has happened? Try it again with an incompressible rod - if the rod
moves, the message reached the far end faster than light. If the rod
doesn't move, where'd the momentum go?
Stupid though experiment.. good for uneducated undergrads...
THERE IS NO SUCH THING AS AN INCOMPRESSIBLE BAR.
--
Gerry Roston (ge...@cmu.edu) | There never did, there never will, and
Field Robotics Center, | there never can exist a parliment, or any
Carnegie Mellon University | description of men, or any generation of
Pittsburgh, PA, 15213 | men, in any country, possessed of the right
(412) 268-3856 | or the power of binding and controlling
| posterity to the `end of time,' or of
The opinions expressed are mine | commanding for ever how the world shall be
and do not reflect the official | governed, or who shall govern it...
position of CMU, FRC, RedZone, | -Every age and generation must be as free
or any other organization. | to act for itself, in all cases, as the
| ages and generations which preceeded it.
| Thomas Paine
Thomas B MacIukenas
>This is one I can't figure out. Consider a steel bar, in free space, a
>long way from anywhere, one light-year long. place a large rocket
>against one end, and turn it on. Let it run for 364 days. Turn it off.
>What has happened? Try it again with an incompressible rod - if the rod
>moves, the message reached the far end faster than light. If the rod
>doesn't move, where'd the momentum go?
Well, there's no such thing as an incompressible rod. No matter what you
build your rod out of, you can never get shock waves to travel faster
than the speed of light within it, so the rod will compress.
--
___-Tom_Maciukenas_(tbmg...@uxa.cso.uiuc.edu)______________________________
Computers hate you.
Do not trust them.
Jack Choquette
|> h...@bcarh11a.BNR.CA (Henry Troup) writes:
|>
|> Well, there's no such thing as an incompressible rod. No matter what you
|> build your rod out of, you can never get shock waves to travel faster
|> than the speed of light within it, so the rod will compress.
|> --
|> ___-Tom_Maciukenas_(tbmg...@uxa.cso.uiuc.edu)______________________________
|>
|> Computers hate you.
|> Do not trust them.
This reminds me of a question I came up with a while ago. Is the
effect of gravaty instantaneous or is it also limited by the speed
of light?
/jack
C, not just a good idea, it's the law.
Matt Austern
> This reminds me of a question I came up with a while ago. Is the
> effect of gravaty instantaneous or is it also limited by the speed
> of light?
This question comes up on sci.physics periodically. A real expert
could doubtless tell you far more than you want to know, but I'll just
make a few comments:
(1) Gravity is very weak, and it's hard to do precision
measurements. Thus nobody has measured the speed of gravity.
You can ask for the predictions of whatever theory of gravity
you happen to believe in, but theories occasionally have a
way of being wrong...
(2) The most widely accepted theory, General Relativity, is quite
complicated. You have to be careful to define the problem
carefully; the answer to this question depends on exactly
what question you ask.
(3) In my opinion, the correct question is: "What is the speed
of gravitational radiation." The predicted answer is that
gravitational radiation propagates at the speed of light.
--
Matthew Austern Just keep yelling until you attract a
(510) 644-2618 crowd, then a constituency, a movement, a
aus...@lbl.bitnet faction, an army! If you don't have any
ma...@physics.berkeley.edu solutions, become a part of the problem!
James Meritt
}In article <1ig87t...@spim.mti.sgi.com> ja...@vermont.mti.sgi.com (Jack Choquette) writes:
}
}> This reminds me of a question I came up with a while ago. Is the
}> effect of gravaty instantaneous or is it also limited by the speed
}> of light?
}
}This question comes up on sci.physics periodically. A real expert
}could doubtless tell you far more than you want to know, but I'll just
}make a few comments:
} complicated. You have to be careful to define the problem
} carefully; the answer to this question depends on exactly
} what question you ask.
I'll buy that. Is he referring to graviton propagation speeds or a time-
space fold. If the latter, I'm not sure at all what he means...
} (3) In my opinion, the correct question is: "What is the speed
} of gravitational radiation." The predicted answer is that
} gravitational radiation propagates at the speed of light.
If it is radiation....
dra...@sscvx1.ssc.gov
>>This is one I can't figure out. Consider a steel bar, in free space, a
>>long way from anywhere, one light-year long. place a large rocket
>>against one end, and turn it on. Let it run for 364 days. Turn it off.
>>What has happened? Try it again with an incompressible rod - if the rod
>>moves, the message reached the far end faster than light. If the rod
>>doesn't move, where'd the momentum go?
>
> You can't make an incompressible rod, precisely for this reason.
>
> Thought experiments allow us to assume things that get around real-world
> engineering limitations, but not to make assumptions which are equivalent to
> violations of physics; in this case, an incompressible rod is one.
>
> So the rocket-powered end is moving and the other isn't. The local motion of
> the matter that is at any moment located in the propagating wave is associated
> with momentum, and this momentum remains the same as the wave progresses.
> --
Stiffness is one of those things one has to watch VERY CAREFULLY when
relativistic effects are to be taken into account. A board sliding along a
table with a hole in it will, in one frame, appear to remain stiff and fall
into the hole. In another frame, the board will appear to sag and dip into the
hole -- resolving the so-called "skateboard and manhole" paradox. The effect is
due to relativity of simultaneity, but it tosses a caution onto the definition
of stiffness and compressibility at relativistic speeds.
An interesting ramification of an accelerating rod is that there is a limit on
how long a rod one can accelerate. The reference is E. Taylor and A. French,
Am.J.Phys. 51 (1983), 889. The paradox comes from the question of how it is a
long rod of length L contracts as it accelerates from rest in time T to a
velocity where it has contracted to L/2. If the front of the rod moves a
distance X in time T, then the rear of the rod will have moved a distance X+L/2
in time T. If L is arbitrarily large, then the rear of the rod can in principle
move faster than the speed of light ==> paradox!
I will try to explain this in as clear English as possible, considering the
cross-posts. The way the contraction happens is as follows. According to
someone riding on the rod, we can accelerate the rod and get it to hold
together by giving the front and back ends equal velocity kicks at the same
time. However, for someone who remains at rest, however, the relativity of
simultaneity predicts that the kick at the back end will happen earlier than
the kick at the front end, the net result being that, as the rod continues to
accelerate in these little steps, the back end "catches up" with the front-end
and the rod contracts. How much earlier the back kick happens compared to the
front kick depends on the length of the rod. This is the fun part. If the rod
is long enough, for given velocity kicks at set intervals at the front end, the
kicks at the back end will pile up on top of each other, so that the limit is
that the back end accelerates to its final velocity !instantaneously! in this
frame of reference. Technically, the limit to the proper acceleration is
a*<=c^2/L* where L* is the proper length of the rod. That is, a rod of length
L* cannot possibly accelerate faster than this without falling apart or
imposing faster-than-light communication. This fact is probably the sternest
filter of unrealistic rocket drives I know.
Paul Draper
University of Texas at Arlington
Mythink, not UTAthink, not SSCthink, not GOVthink.
John F Blanton
John Blanton
bla...@lobby.ti.coM
Jason F Ralph
:
: Is the
: effect of gravaty instantaneous or is it also limited by the speed
: of light?
Well, if you can answer that one, you should book your flight to Sweden
straight away!
(To give a simple answer: Yes.)
(The gravitational interaction is mediated by 'gravitons' which are spin
2 massless particles which do indeed travel at the speed of light...
which gives rise to the inverse square law for gravitation interactions
amongst other things... according to the currently 'trendy' quantum
field theories of gravity.)
(However, these theories cannot be renormalised, i.e. they don't add up
properly, and are so full of holes (black, white or otherwise) that few
people really beleive them... hence the attraction for superstring
theories which DO add up... a bit strangley for my taste... but they
don't actually mean anything!)
Splottie-kins (Strange).
Richard A. Schumacher
incompressible bar in a relativity thought experiment, because
"incompressible" means (among other things) "infinite speed of
sound". Note that it can still make perfect sense to suppose
an incompressible bar in many Newtonian (non-relativistic)
mechanics thought experiments, if one is not concerned with
the speed of momentum transfer. The fact that such a bar is
impossible to make is irrelevent to BOTH kinds of thought
experiment.
Dan'l DanehyOakes
>This is one I can't figure out. Consider a steel bar, in free space, a
>long way from anywhere, one light-year long. place a large rocket
>against one end, and turn it on. Let it run for 364 days. Turn it off.
>What has happened? Try it again with an incompressible rod - if the rod
>moves, the message reached the far end faster than light. If the rod
>doesn't move, where'd the momentum go?
It's real simple.
There's no sech kind of animal as an incompressible rod.
The far end of the rod will not have begun to move at T+364; indeed, it will
probably not begin to move until T>>+365. Momentum will be dispersed along
the length of the rod in compression waves. What is the speed of sound in
the material the rod is made of? Divide C by that speed and you've got the
factor for how long it will be before the far end of the rod moves.
I'm your only friend. I'm not your only friend,
but I'm your little glowing friend, but actually
I'm not your friend. But I am
--TMBG
Dan'l Danehy-Oakes, Net.Roach
My opinions do NOT represent Pacific Bell,
Professional Development, or anyone else.
But I'm willing to share.
Daniel Seeman
>To be more precise, it makes no sense to talk about an
>incompressible bar in a relativity thought experiment, because
>"incompressible" means (among other things) "infinite speed of
>sound". Note that it can still make perfect sense to suppose
Good point ("...infinte speed of sound..."). But thought experiments need not
be real to be usefull.
dks.
Blair P. Houghton
Ol' Dr. Park, back at the U. of Md., got tenure by performing
an experiment which changed the law of gravity from GmM/r^2.0000...
to GmM/r^2.000000...
His apparatus was a SQUID in a vibration-damping container
hung about 3 m away from a gigantic pendulum (1500 lbs of
steel or aluminum, I forget which).
[His point was that the inverse-square law means that gravity
is solenoidal, so if you measure deformation of six squids,
each on the face of a cube, they integrate to zero, which
they did, to two more significant digits than anyone's ever
measured such a thing before...the pendulum was used to
give a large amplitude to the useful components of the
gravity so that noise in the system could be easily
filtered out.]
The question is, could you measure the phase between two
measurements taken simultaneously at 3 m and 4 m, given the
same apparatus? The amplitudes at this range are in the
ratio 9:16, or about 1:2, which would leave the smaller of
them well within the measurement range of Dr. Park's squids
(if his oscilloscope was to have been believed). We could
then get a measure of the time taken to travel between them
by the gravitational intensity fluctuation as it propagated
away from the pendulum.
If we aim for a ballpark of 3e8 m/s, then 1m is about 3e-9
s, or about three nanoseconds (Grace Hopper would confirm
this, God rest her code), which is pretty bloody huge by
modern timing standards. The computer under your fingers
is full of transistors which perform boolean operations
faster than that. We could easily get lab timers with
sub-picosecond resolution and give ourselves 3-4
significant digits of precision. If we're really nice
about it, we could probably get femtosecond-resolution
differential timers and our pictures in the Encyclopaedia
Brittanica.
Hay, who's got a DARPA grant form handy?
--Blair
"The things you pick up from a
lifetime of dumpster diving."
Eric S. Raymond
> This is one I can't figure out. Consider a steel bar, in free space, a
> long way from anywhere, one light-year long. place a large rocket
> against one end, and turn it on. Let it run for 364 days. Turn it off.
> What has happened? Try it again with an incompressible rod - if the rod
> moves, the message reached the far end faster than light. If the rod
> doesn't move, where'd the momentum go?
>
> Now shrink the system to one light-second long, and run the engine in
> half second bursts. All the same effects have to be happening.
>
> Now shrink the system to one meter and run the "engine" at 600 MHz -
> again, the same 'delay' effects are there.
driving at than I'd thought of.
> But Davis didn't really account for the Dean Drive - partly because the
> device Dean demo'd wasn't the one in the patent drawings, and he
> wouldn't let anyone closely examine his model. Dean was a classic
> crackpot, as Harry Stine admits. Stine did a followup article on Davis
> mechanics and some actual experiments that did have unexpected results.
> Like a simple mechanical system with an unexplained 3 degree phase shift.
Yup. This is the article I read, I think; that's where I got the bit
about the artillery shells.
There is real physics here that no one is following up. It's sad.
Eccles
>In article <1992Dec30.0...@blaze.cs.jhu.edu>,
>arro...@jyusenkyou.cs.jhu.edu (Ken Arromdee) writes:
>|>But the statement you just made about objects isn't true. Action and
>reaction
>|>happen simultaneously; or rather, action and reaction happen at any point
>|>simultaneously.
>This is one I can't figure out. Consider a steel bar, in free space, a
>long way from anywhere, one light-year long. place a large rocket
>against one end, and turn it on. Let it run for 364 days. Turn it off.
>What has happened? Try it again with an incompressible rod - if the rod
>moves, the message reached the far end faster than light. If the rod
>doesn't move, where'd the momentum go?
>Now shrink the system to one light-second long, and run the engine in
>half second bursts. All the same effects have to be happening.
>Now shrink the system to one meter and run the "engine" at 600 MHz -
>again, the same 'delay' effects are there.
huh? Whats the problem? When you push against the millions of atoms
at one end of the bar, they start to move, and their electron shells
start to influence others along the bar, etc cetra.....
so the bar shrinks.... big deal, this is one sodding heavy pieve
of steel, its billions of kms long......
I may be being completely iggerant, but it wouldn't be the first time
Eccles :)
Blair P. Houghton
>In <18...@bcars664.bnr.ca> Henry Troup wrote:
>> This is one I can't figure out. Consider a steel bar, in free space, a
>> long way from anywhere, one light-year long. place a large rocket
>> against one end, and turn it on. Let it run for 364 days. Turn it off.
>> What has happened? Try it again with an incompressible rod - if the rod
>> moves, the message reached the far end faster than light. If the rod
>> doesn't move, where'd the momentum go?
Someone mentioned "compression waves," and I think
everyone's explained that "incompressibility" is
impossible. What I'd like to remind everyone of is that
solid materials of supraatomic extent are mechanically
connected by electromagnetic forces (the rigid equilibrium
of systems of nuclei bonded by molecular electron clouds
and intertwined by metallurgy, crystallization, etc.).
In the absence of dispersion, the best you can do is to
line up atoms and whack one with a scanning-tunnelling
microscope to see how fast the electromagnetic disturbance
takes to get to the other end.
Any mechanical (acoustical, momentum) disturbance will take
far longer, since it involves the motion of the nuclei.
Assume though that you can create a theoretically
"incompressible" rod (what's a good gedankenexperiment
without an assumption to be countervened?).
The frontier of impulse from the disturbance at one
end would propagate with the frontier of relativistic
energy (which moves at c).
The rod's elements of mass can not be accelerated if the
elements further down the rod will not move (this is the
definition of incompressible rigidity), so they will not
move until the frontier of impulse reaches the last element.
So, slice the rod into a line of short, incompressible
cylinders, and throw one at one end. After the
relativistically correct time the last will pop off the
other end. If they're suspended on strings it will swing
back, causing the reverse reaction, and we can be
mesmerized by it for centuries.
During the time between visible motions of the two
cylinders at the end, where is the momentum? It is
somewhere in the rod, existing as pure momentum without
displacement (in the real version of this toy it exists as
minute deformations in the hanging balls, propagating as an
acoustical wave; is it correct then that incompressible
balls could not move because they could not propagate a
real impulse from one end to the other?). The cylinders
between the ends act as though they have infinite mass.
But if the impulse is constant at one end, then the
frontiers of impulse are constant accross the length of the
rod behind the initial frontier. When the initial frontier
reaches the end, the final cylinder moves in response to
it. Once it is out of the way, the next frontier of
impulse accelerates the penultimate cylinder.
This produces a backward-propagating frontier of
acceleration.
If the rod is whole, we have simply a frontier of impulse
that propagates, reaches the other end, and is reflected,
because the end can not move since it is tied to the rest
of the rod, which is not moving.
If the impulse is constant, then it reaches the end,
reflects, and propagates backward, cancelling the forward
moving frontiers of impulse. When it reaches the beginning,
it cancels the impulse generated by the external force, or
reflects again and doubles the frontier of impulse the external
force continues to generate.
It seems that relativity prevents an incompressible object
from accelerating when acted on by point forces. (A field
of force acting all along the length of the rod should
accelerate it, still).
This should work on any scale, so even our incompressible
cylinders should not have moved.
"Where does the momentum go?" It goes into the effectively
infinite mass of the incompressible object, which accelerates
at an effectively infinitesimal rate, and dp/dt = f = ma = oo * 0
can be anything you want it to be. No energy is transferred
between the rocket and the rod, apparently.
[This also spannerworks our ideas about incompressible
fluids, btw.]
So how does any real, "rigid" object accelerate when it's
pushed? (Someone already mentioned compression waves;
that's all this is.) Each atom in turn pushes on the
successive atom, propagating the frontier of momentum at
the speed given by Young. Frontiers of momentum continue
to arrive if the force is continuous, and the object moves
all in one direction. If the force is momentary, the
object moves only a little bit and vibrates a lot. How can
a unit (dirac-like) impulse make the entire object move?
Like a snake. When the compression reaches the end, the
last atom moves outward into space and pulls on the one
before it. The frontier of momentum is reflected and
propagates backward, but is now a frontier of negative
momentum, since the atoms are pulling.
This is different from the reflected frontiers of momentum
in the incompressible rod, which were reflected like a ball
off a wall, causing the frontier to reverse direction but
remain a pushing force (I have a really bad analogy
involving ornery drivers at a long stoplight, here, but
I'll omit it), cancelling the forward push.
>Thank you, Mr. Troup. This is a better way of making the point I'd been
>driving at than I'd thought of.
What point is that? It seems to depend on a fallacious
assumption. To wit, that incompressible objects *can* be
accelerated by point forces.
>> But Davis didn't really account for the Dean Drive - partly because the
>> device Dean demo'd wasn't the one in the patent drawings, and he
>> wouldn't let anyone closely examine his model. Dean was a classic
>> crackpot, as Harry Stine admits. Stine did a followup article on Davis
>> mechanics and some actual experiments that did have unexpected results.
>> Like a simple mechanical system with an unexplained 3 degree phase shift.
>
>Yup. This is the article I read, I think; that's where I got the bit
>about the artillery shells.
>
>There is real physics here that no one is following up. It's sad.
Que es que c'est, "Dean Drive?"
--Blair
"And here I always thought I
was an irresistable force..."
Eric S. Raymond
> What point is that? It seems to depend on a fallacious
> assumption. To wit, that incompressible objects *can* be
> accelerated by point forces.
I wasn't focusing on the "incompressible" case in Mr. Troup's posting at all,
though everyone else seems to have. My point (which you've made in even more
detail) is that there is a measurable interval during which the object's
behavior is non-Newtonian.
> Que es que c'est, "Dean Drive?"
A complicated mechanical device which supposedly functioned as a "reactionless
drive". It seems pretty clear (by experimental test) that the Dean Drive did
not work; however, Davis, Stine & Victory (who *did* the experimental test)
developed some theory which convinced them that a variation might, and even
got some experimental results which fit the theory but *not* conventional
Newtonian mechanics. At about that point, the research was defunded.
Blair P. Houghton
>In <1incj8...@chnews.intel.com> Blair P. Houghton wrote:
>> What point is that? It seems to depend on a fallacious assumption.
>> To wit, that incompressible objects *can* be accelerated by point forces.
>
>I wasn't focusing on the "incompressible" case in Mr. Troup's posting at all,
>though everyone else seems to have. My point (which you've made in even more
>detail) is that there is a measurable interval during which the object's
>behavior is non-Newtonian.
To whom? If the bar is compressible, then anyone at the
rocket end can't see that the other end isn't moving in
response to the force, and anyone at the other end can't
see the force until it gets there; and if the bar is
incompressible it certainly isn't Newtonian, and won't move
in response to any force that it must propagate itself.
But, like Howard Hughes learned with the hydraulics in the
Spruce Goose, your physics professor was screwing you blind
talking about "incompressible" materials.
>> Que es que c'est, "Dean Drive?"
>
>A complicated mechanical device which supposedly functioned as a "reactionless
>drive".
Ah. And how did it supposedly accomplish this?
--Blair
"If perpetual motion were
legal, it would kill more
people than smoking."
Hartmut Frommert
> You can ask for the predictions of whatever theory of gravity
> [...]
> (2) The most widely accepted theory, General Relativity, is quite
> complicated.
> [...]
> gravitational radiation propagates at the speed of light.
If I'm not misinformed, there's also no *special* relativistic theory for
gravity where gravity propagates faster.
-
Hartmut Frommert <phf...@nyx.uni-konstanz.de>
Dept of Physics, Univ of Constance, P.O.Box 55 60, D-W-7750 Konstanz, Germany
-- Eat whale killers, not whales --
Gerry Roston
We have all (well almost all) agreed that the original question was
silly because there is no such thing as an incompressible substance.
But is that true? When a star colapses and becomes a black hole, what
is the density of the material? Isn't this theorectically a mass of
nuclei smushed up against ecah other? If so, how would one compress
this any further? Of course, it is probably meaningless to think
about applying a force to such an object...
--
Gerry Roston (ge...@cmu.edu) | We hold these truths to be self-evident, that
Field Robotics Center, | all men are created equal, that they are
Carnegie Mellon University | endowed by their Creator with certain
Pittsburgh, PA, 15213 | unalienable Rights, that among these are
(412) 268-3856 | Life, Liberty, and the pursuit of Happiness.
| That to secure these rights, Governments are
The opinions expressed are mine | instituted among Men, deriving their just
and do not reflect the official | powers from the consent of the governed.
position of CMU, FRC, RedZone, | That whenever any Form of Government becomes
or any other organization. | destructive of these ends, it is the Right of
| the People to alter or to abolish it...
| Thomas Jefferson
Blair P. Houghton
>We have all (well almost all) agreed that the original question was
>silly because there is no such thing as an incompressible substance.
Well, I wouldn't call it silly; just self-contradictory.
>But is that true? When a star colapses and becomes a black hole, what
>is the density of the material? Isn't this theorectically a mass of
>nuclei smushed up against ecah other? If so, how would one compress
>this any further? Of course, it is probably meaningless to think
>about applying a force to such an object...
Because of the curvature of space in and around such
a thing, you can't really measure a "density."
Nuclei would be rent asunder, and protons and neutrons
would probably get squashed into quarks. There would be
electrons (and if electrons, oxygen, which means air, so we
could breathe...oops; slipped into quayle-mode there for a
second...).
I guess the wavefunctions themselves could get squeezed
down to point-like objects (if they don't collapse ha ha :-)).
There's also the view that the innards of a black hole are
collapsing ad infinitum, and actually _decreasing_ in
density in its own frame, since space is being sucked into
the hole at a greater rate than matter is. Inside the
hole, a particle observes the boundary to be receding; the
hole seems to be expanding; much like our own universe;
which now we can conjecture could be the innards of just
such a black hole.
What was the question, again?
--Blair
"This posting needs more
CAPITALIZED words..."
Erik Max Francis
> We have all (well almost all) agreed that the original question was
> silly because there is no such thing as an incompressible substance.
> But is that true? When a star colapses and becomes a black hole, what
> is the density of the material? Isn't this theorectically a mass of
> nuclei smushed up against ecah other? If so, how would one compress
> this any further? Of course, it is probably meaningless to think
> about applying a force to such an object...
Talking about the density of black holes is rather meaningless. Neutron
stars _do_ have the density of nuclear material, but can be comrpessed
further (though not easily). After all, during the neutron star's
formation, the core collapse causes the core to be compressed to up to
5 or so times the density of nuclear material, which is pretty durn high.
____ Erik Max Francis -- ..!apple!uuwest!max -- m...@west.darkside.com __
\ / 1070 Oakmont Dr. #1 San Jose, CA 95117 37 20 N 121 53 W ACGT / \
\/ Like strategic interstellar conquest games? Ask about UNIVERSE! \__/
Omnia quia sunt, lumina sunt. All things that are, are lights.
Jason F Ralph
about black holes I thought that you might like to know that according
to Einstein's Theory of General Relativity... they don't exist. (Or,
more precisely, they cannot form... stuff/matter falling into a black
hole NEVER reaches it... time dilation and so forth.)
The main problem comes from the fact that you cannot reconcile
Einsteinian gravity with quantum field theory... yet! Maybe in the
future it will be possible to marry quantum theories with classical
theories, but it's not possible yet! All this talk about matter in black
holes is complete tosh... we have no idea what happens when you compress
things beyond the nucleic limit. In any case, if you believe QFT's a
quark-electron 'blob' (NOT necessarily a black hole) is still not
incompressible since the speed of 'signal' propagation is still given by
the speed of light... Assuming that the gauge symmetry has been broken,
then only the photon and graviton are massless, the weak bosons and the
gluons have mass => 'signal' propagates slower than the speed of
light... The dominate 'signal' may still be given by the colour force
since this the strongest interaction, but this is still limited by 'c'.
Splottie-kins (respectable and responsible).
Matt Austern
> I really DO hate to spoil the party, but if you want to argue the toss
> about black holes I thought that you might like to know that according
> to Einstein's Theory of General Relativity... they don't exist.
Not true; there's been a lot of work on the question of whether or not
black holes can form. It can be rigorously proved (by Penrose and
Hawking, among others) that certain initial conditions will lead to
black hole formation. In the case of more realistic initial
conditions, the situation is somewhat less certain, but the general
consensus is that black holes can form.
Chandrasekhar said "People who are skeptical about black holes think
they're being conservative, but they really aren't. They're being
radical."
SCOTT I CHASE
>I really DO hate to spoil the party, but if you want to argue the toss
>about black holes I thought that you might like to know that according
>to Einstein's Theory of General Relativity... they don't exist. (Or,
Matt Austern already addressed this part, so I'll just skip it.
>theories, but it's not possible yet! All this talk about matter in black
>holes is complete tosh... we have no idea what happens when you compress
>things beyond the nucleic limit. In any case, if you believe QFT's a
I don't know why you say that. We have plenty of experience squeezing
nuclear matter to well more than an order of magnitude higher density
than normal nuclear density. Nothing special happens, necessarily -
it's an open question whether you can create circumstances under which
such compression will lead to quark-gluon plasma formation.
>quark-electron 'blob' (NOT necessarily a black hole) is still not
>incompressible since the speed of 'signal' propagation is still given by
>the speed of light... Assuming that the gauge symmetry has been broken,
>then only the photon and graviton are massless, the weak bosons and the
>gluons have mass => 'signal' propagates slower than the speed of
>light... The dominate 'signal' may still be given by the colour force
>since this the strongest interaction, but this is still limited by 'c'.
True, but irrelevant. Whether or not signals travel at c inside the
hole is immaterial, and not at all clear to me anyway. Black holes are
strange objects. In any event, this does not preclude a small object,
like a nucleus, from being squeezed so tightly that it becomes a
microscopic black hole - for the short time before it evaporates. Many
such micro black holes are believed to have been formed in the Big Bang.
-Scott
--------------------
Scott I. Chase "It is not a simple life to be a single cell,
SIC...@CSA2.LBL.GOV although I have no right to say so, having
been a single cell so long ago myself that I
have no memory at all of that stage of my
life." - Lewis Thomas
Robert Coe
> We have all (well almost all) agreed that the original question was
> silly because there is no such thing as an incompressible substance.
> But is that true? When a star colapses and becomes a black hole, what
> is the density of the material? Isn't this theorectically a mass of
> nuclei smushed up against ecah other? If so, how would one compress
> this any further? Of course, it is probably meaningless to think
> about applying a force to such an object...
Hardly meaningless, given that a black hole can presumably be a member of
an orbiting binary system. But to me the biggest problem with the initial
assumption isn't the incompressibility of the rod, but rather that a rigid
rod a light year in length could exist at all without, for example, being
torn apart by tidal forces. (Yes, I know that the assumption was that the
rod be remote from gravitational attractors, but a rod that long at rest in
space should see different spacetime curvature at its two ends. Or to put
it another way, I don't think you can find a way to orient a rod that long
while pretending that its two ends are in the same reference frame.)
___ _ - Bob
/__) _ / / ) _ _
(_/__) (_)_(_) (___(_)_(/_______________________________________ b...@1776.COM
Robert K. Coe ** 14 Churchill St, Sudbury, Massachusetts 01776 ** 508-443-3265
Erik Max Francis
> I really DO hate to spoil the party, but if you want to argue the toss
> about black holes I thought that you might like to know that according
> to Einstein's Theory of General Relativity... they don't exist. (Or,
> more precisely, they cannot form... stuff/matter falling into a black
> hole NEVER reaches it... time dilation and so forth.)
Sorry, no. From an external frame of reference -- that is, from someone
outside the influence of the black hole -- a particle never reaches the
event horizon. But from the point of view of someone actually falling
into the black hole, the particle passes through the event horizon
without trouble; it's the _singularity_ that the particle never reaches.
Take a spaceman falling into a black hole. His friends in the space
station orbiting safely out of the static limit and accretion disk are
watching. He sends a signal back to the station to let them know what's
going on; he sends a regular pulse of coherent light every second.
He starts falling. As he gets closer to the event horizon, his
colleagues note that his signals are getting farther and farther apart,
weaker and weaker, and the frequency of the light he's sending is getting
more and more redshifted. They notice that the limit is the event
horizon -- as the spaceman's position approaches the event horizon, the
length of time between pulses approaches infinity.
But that's only from the station's point of view. From the spaceman's
point of view, things are rather different. We'll assume that the tides
from the black hole are not too severe, so the spaceman can actually
experience what's going on. (This isn't too unreasonable an assumption
near the event horizon if the black hole is sufficiently large.)
He starts falling. He sends his pulse every second. Since it's being
sent every second from his frame of reference, he never notices anything
strange about the time between pulses (until the tides wreck him and his
beacon).
If the black hole is large enough, the spaceman won't notice anything
strange _at all_ as he passes the event horizon. Now he is inside the
black hole, and can never get out. As he falls further and further, the
gravitational pull increases and the tidal forces increase. Note that he
can still watch what's going on _outside_ of the black hole; the black
hole is only a one-way membrane, pointing toward him. Nothing
spectacular (neglecting tidal forces which would have killed him long
ago) seems to be happening, but as he looks up (out of the black hole),
he notices that strange things are happening. He sees the Universe sped
up. He sees all the stars die one by one, he sees the galaxies disappear
beyond the edge of visibility . . . and then he dies as the black hole
spits him out, particle by particle, as Hawking radiation.
This of course assumes that the Universe is open. If it isn't, then he'd
see the galaxies get closer and closer, the stars boil, and the Big
Crunch.
wood,christopher
>Sorry, no. From an external frame of reference -- that is, from someone
>outside the influence of the black hole -- a particle never reaches the
>event horizon.
No, the particle has (ignoring quantum effects here...) a position and a
velocity. The particle is accelerated toward the black hole, and an
observer can identify a point in time when the particle crosses the
event horizon.
[trimmed. Spaceman sends signals every second to outside observers]
>He starts falling. As he gets closer to the event horizon, his
>colleagues note that his signals are getting farther and farther apart,
>weaker and weaker, and the frequency of the light he's sending is getting
>more and more redshifted.
Yup. But the series does not converge to zero in infinite time, it does
so in finite time - when the (point-sized) astronaut crosses the event
horizon.
If you plot position vs. time, the position is accelerating towards the
event horizon. If you plot frequency vs. time, the frequency goes down
as the astronaut gets deeper into the black hole's potential well, and
crosses zero when the astronaut crosses the event horizon.
>But that's only from the station's point of view. From the spaceman's
>point of view, things are rather different.
>He starts falling. He sends his pulse every second. Since it's being
>sent every second from his frame of reference, he never notices anything
>strange about the time between pulses (until the tides wreck him and his
>beacon).
> ____ Erik Max Francis -- ..!apple!uuwest!max -- m...@west.darkside.com __
Chris
--
Chris Wood Bellcore ...!bellcore!prefect!ccw
or c...@prefect.cc.bellcore.com
Brad Templeton
have (really an energy) if one applies no more thrust.
One can, of course, escape from the Earth's gravitational field at one
mile an hour, if under a constant force equal to the gravitational force.
Black holes are really a horse of a different colour, although they have
no colour, and no hair.
--
Brad Templeton, ClariNet Communications Corp. -- Sunnyvale, CA 408/296-0366
Thomas B Maciukenas
>ge...@cmu.edu (Gerry Roston) writes:
>> We have all (well almost all) agreed that the original question was
>> silly because there is no such thing as an incompressible substance.
>> But is that true? When a star colapses and becomes a black hole, what
>> is the density of the material? Isn't this theorectically a mass of
>> nuclei smushed up against ecah other? If so, how would one compress
>> this any further? Of course, it is probably meaningless to think
>> about applying a force to such an object...
What you described isn't a black hole. In fact, it's not even a neutron
star. Continue compressing it until all the protons turn to neutrons and
you'll have a neutron star. Keep compressing and eventually the neutrons
will collapse and everything will shrink down to a single point -- a
singularity -- and THEN you'll have a black hole. Of course, it is
meaningless to talk about compressing a point, but anything else is
compressible.
--
_-Tom_Maciukenas_(to...@crhc.uiuc.edu)_________________________
"To live reflected in a spoon,
makes it too hard to stay in tune." - Greg Lake "Huh?" - Me
Jason F Ralph
: In article <1993Jan12.1...@syma.sussex.ac.uk>, jas...@syma.sussex.ac.uk (Jason F Ralph) writes...
:
: >theories, but it's not possible yet! All this talk about matter in black
: >holes is complete tosh... we have no idea what happens when you compress
: >things beyond the nucleic limit. In any case, if you believe QFT's a
:
: I don't know why you say that. We have plenty of experience squeezing
: nuclear matter to well more than an order of magnitude higher density
: than normal nuclear density. Nothing special happens, necessarily -
: it's an open question whether you can create circumstances under which
: such compression will lead to quark-gluon plasma formation.
Sorry, I should have been more specific... We don't know what happens to
matter in regions of extreme curvature, such as that which exists in a
black hole. (Although, I'm not sure that it is yet possible to 'squeeze'
nuclei directly... You can smash them together really hard, but I don't
know about 'squeezing'.)
: >quark-electron 'blob' (NOT necessarily a black hole) is still not
: >incompressible since the speed of 'signal' propagation is still given by
: >the speed of light... Assuming that the gauge symmetry has been broken,
: >then only the photon and graviton are massless, the weak bosons and the
: >gluons have mass => 'signal' propagates slower than the speed of
: >light... The dominate 'signal' may still be given by the colour force
: >since this the strongest interaction, but this is still limited by 'c'.
: True, but irrelevant. Whether or not signals travel at c inside the
: hole is immaterial, and not at all clear to me anyway. Black holes are
: strange objects. In any event, this does not preclude a small object,
: like a nucleus, from being squeezed so tightly that it becomes a
: microscopic black hole - for the short time before it evaporates. Many
: such micro black holes are believed to have been formed in the Big Bang.
I was merely trying to point out that a quark-electron 'blob' would
still not be incompressible... don't blame me, someone else suggested
it... As for the microscopic black hole thing, true but equally
irrelevant. The main problem would be trying to squeeze a nucleon to
such a size. If any did form in the Big Bang, they would be long gone by
now, they evaporate very very quickly.
Splottie-kins (Sceptic).
Jason F Ralph
: jas...@syma.sussex.ac.uk (Jason F Ralph) writes:
:
: > I really DO hate to spoil the party, but if you want to argue the toss
: > about black holes I thought that you might like to know that according
: > to Einstein's Theory of General Relativity... they don't exist. (Or,
: > more precisely, they cannot form... stuff/matter falling into a black
: > hole NEVER reaches it... time dilation and so forth.)
:
: Sorry, no. From an external frame of reference -- that is, from someone
: outside the influence of the black hole -- a particle never reaches the
: event horizon. But from the point of view of someone actually falling
: into the black hole, the particle passes through the event horizon
: without trouble; it's the _singularity_ that the particle never reaches.
[stuff about spaceman deleted]
True. Take a simple case, one black whole and an otherwise flat
universe. According to the frame of reference of someone falling into
the black hole, they do pass into the black hole... and they don't even
notice doing it (ignoring 'tides'... c.f. other article in thread). BUT,
as they look out into the rest of the universe they see it dying around
them. The stars go nova/super-nova/burn-out, everything fizzles away to
nothing. The outside world never sees things falling into a black hole,
we are all dead before that happens. Hence, the black hole in
Einsteinian gravity will not form. It CAN be put in as an intial
condition, but it cannot form from an otherwise 'normal' space. (Black
hole formation is not totally ruled out when you include quantum effects
though... but, as I've already said, there is no consistent way to do
that as yet.)
Love and hugs from,
Splottie-kins (Cynic).
Matt McIrvin
>Chandrasekhar said "People who are skeptical about black holes think
>they're being conservative, but they really aren't. They're being
>radical."
A few days ago there was a long opinion piece in the New York Times-- I
think it was by a professor of some social science and was called
"History, Wild History"-- which was mostly about how quickly ideological
fads and visions of the future pass; mostly, a nice, sober reality check
of an article. He had, though, a list of "imbecilic ideas of a worn-out
century" that will surely pass as well, and black holes were figured
prominently. The others were mostly pop-cultural buzzwords of some sort
or another.
I thought that was very strange. Surely black holes are either real, or
they aren't; and the balance of evidence seems to be in favor of their
existence lately. I thought it odd that this guy believed he had a better
source of knowledge than the astronomers.
He might, of course, have meant black holes as a cultural phenomenon, a
hip metaphor for death, horror and the unknown. In that case, he's
probably right-- that business peaked sometime around 1980.
--
Matt McIrvin I read Usenet just for the tab damage!
==== ======= = ==== ====== ==== === === === =======
Matt McIrvin
>True. Take a simple case, one black whole and an otherwise flat
>universe. According to the frame of reference of someone falling into
>the black hole, they do pass into the black hole... and they don't even
>notice doing it (ignoring 'tides'... c.f. other article in thread). BUT,
>as they look out into the rest of the universe they see it dying around
>them. The stars go nova/super-nova/burn-out, everything fizzles away to
>nothing.
No, they don't. This is a common misconception; one cosmologist even
voices it in Errol Morris's film about Hawking.1 An observer falling
into a black hole will not see the universe die first. If you examine the
past light-cones of an observer falling into a black hole, you will
find that they do *not* encompass events arbitrarily far into the future
at infinity, not even when the observer is at the event horizon.
The *future* light-cones do this, which is why someone outside the hole sees
the fall occur asymptotically slowly. But that doesn't mean that the
"black hole" is never observed to form. Light coming from the falling
object gets dimmer and dimmer to an outside observer, and this happens
very rapidly; in addition, since light is actually made up of discrete
photons, the *last* photon emitted before the object disappears through
the horizon will, in general, reach an outside observer at a finite time
(in all physically reasonable situations, this occurs in a short time).
So even though an examination of the future light-cones leads one to
believe that an outside observer would see a "frozen star" overhung with
frozen infalling debris, it would actually look quite featureless and
quite black-- the term "black hole" is justified.
The discussions about this in Misner, Thorne and Wheeler's text _Gravitation_
are quite illuminating; they're (IMHO) the best part of the book.
They go into further detail about, for instance, what would happen if you
tried to swoop down and pick up some of the "frozen" material (it wouldn't
work).
1 It's quite possible that some of the context was omitted from that scene,
and that he was actually talking about the "inner horizon" of a
Reissner-Nordstrom hole, at the entrance to the vaunted wormhole; something
of this variety actually happens there, and it's theorized that the
"infinite blueshift" involved would cause the wormhole to collapse. If
so, then he knew what he was talking about.
Benjamin Weiner
>A few days ago there was a long opinion piece in the New York Times-- I
>think it was by a professor of some social science and was called
>"History, Wild History"-- which was mostly about how quickly ideological
>fads and visions of the future pass; mostly, a nice, sober reality check
>of an article. He had, though, a list of "imbecilic ideas of a worn-out
>century" that will surely pass as well, and black holes were figured
>prominently. The others were mostly pop-cultural buzzwords of some sort
>or another.
I think the author was John Lukacs (no relation to the more famous
Georg Lukacs). And yes, I noticed the presence of black holes on his
list. In fact, there was something else in the list that ticked me off,
but I can't remember if it was a scientific term or not.
>I thought that was very strange. Surely black holes are either real, or
I thought it was very stupid, for reasons explained below.
>they aren't; and the balance of evidence seems to be in favor of their
>existence lately. I thought it odd that this guy believed he had a better
>source of knowledge than the astronomers.
>He might, of course, have meant black holes as a cultural phenomenon,
>hip metaphor for death, horror and the unknown. In that case, he's
>probably right-- that business peaked sometime around 1980.
He did mean that, I think, but in a less innocuous sense. I saw the thrust
of the article as being "well, the 20th century is finally just about over,
and as we say goodbye to it we can say goodbye to the noxious idea of
'progress into the future' and all that." While I agree that the
unreserved acceptance of Progress as Good has led to all sorts of bad
things, Buck Rogers science, etc. etc., Lukacs seemed to be engaging
in what I call Golden Age-ism: nostalgia for an earlier age in which
everything was simpler and more wholesome (as if science could be
blamd for all of WWII, not just the Bomb. And so on.)
Given that, his inclusion of Black Holes in his list was a rejection of
technological progress and science in general as "inhuman" to at least
some extent. My retort to people who say this is usually, "All right,
did you use a word processor to write your article?" (By the way, this
interpretation of his article is fairly reasonable and not my fevered
imagining.) And what about indoor plumbing?
I apologize for ranting on at length about this, but I think it's
important to recognize this style of nostalgia as nostalgia, not
history. And to see that anti-scientism is arising these days from
the "good old days" style conservatives as much as the (more often
blamed) environmental/peacenik liberals.
-Ben
Erik Max Francis
> The Earth has an escape velocity of about 25,000 miles per hour. This
> doesn't mean that you have to attain this velocity to escape the Earth
> forever (you can use a constant thrust that just counteracts the Earth's
> pull and adds a tiny additional velocity). The escape velocity is only
> for (I think) ballistic trajectories. Right?
Not really. First of all, escape velocity deals with _velocity_, not
acceleration (you mentioned thrust). The escape velocity for any mass is
a function of height; the higher you are above the Earth, the smaller the
escape velocity you'll need. At the surface it's something like 11 km/s.
This means that, at that height, you would need a velocity of 11 km/s or
more to escape Earth's gravity well forever. If you're applying thrust,
things are a little different, because you're going to be changing your
velocity as you're changing your height, making the situation rather more
complicated. But to escape the Earth's gravity well, at some point you
_must_ have the escape velocity for the height at which you are.
If you don't, you'll eventually fall back down.
> I take it then the the escape velocity for a black hole isn't really
> the same thing since I know that nothing can escape from a black hole,
> even if you use a constant thrust, etc (ignore Hawking radiation)?
> Am I right?
I see the point you're trying to make, but things are a little different
with a black hole. The event horizon of the black hole is defined as the
radius around the black hole where the escape velocity is lightspeed, c.
If you are to fall into this (assuming you survive electromagnetic fields
and tidal forces -- which is possible for large black holes), that means
that, no matter what, you'd have to have an escape velocity of _at least_
c to escape. (The closer you are to the center of the black hole -- the
singularity -- of course, the greater the escape velocity required.) No
material object can travel at or beyond the speed of light, so you can
never escape. Ever.
____ Erik Max Francis -- ..!apple!uuwest!max -- m...@west.darkside.com __
Erik Max Francis
> Black holes are really a horse of a different colour, although they have
> no colour, and no hair.
Yep. The only things they take with them are mass, charge, and angular
momentum. :-)
Steinn Sigurdsson
Erik Max Francis (m...@west.darkside.com) wrote:
: jas...@syma.sussex.ac.uk (Jason F Ralph) writes:
: > I really DO hate to spoil the party, but if you want to argue the toss
: > about black holes I thought that you might like to know that according
: > to Einstein's Theory of General Relativity... they don't exist. (Or,
: > more precisely, they cannot form... stuff/matter falling into a black
: > hole NEVER reaches it... time dilation and so forth.)
: Sorry, no. From an external frame of reference -- that is, from someone
: outside the influence of the black hole -- a particle never reaches the
: event horizon. But from the point of view of someone actually falling
: into the black hole, the particle passes through the event horizon
: without trouble; it's the _singularity_ that the particle never reaches.
[stuff about spaceman deleted]
True. Take a simple case, one black whole and an otherwise flat
universe. According to the frame of reference of someone falling into
the black hole, they do pass into the black hole... and they don't even
notice doing it (ignoring 'tides'... c.f. other article in thread). BUT,
as they look out into the rest of the universe they see it dying around
we are all dead before that happens. Hence, the black hole in
Einsteinian gravity will not form. It CAN be put in as an intial
condition, but it cannot form from an otherwise 'normal' space. (Black
Sorry, mate, but you're wrong. You are neglecting the mass of the
object falling in, it contributes to the local curvature. One way of
looking at it is that the horizon extends out to envelope the
infalling object as it approaches the horizon. See "The Membrane
Paradigm" by Thorne et al for a formal treatment (Sussex Library
has it), or, if you've taken GR to the level of Barrow's course or
beyond, go talk to John Barrow - he'll set you straight.
* Steinn Sigurdsson Lick Observatory *
* ste...@lick.ucsc.edu "standard disclaimer" *
* The laws of gravity are very,very strict *
* And you're just bending them for your own benefit - B.B. 1988*
Andrew F. Nelson
>br...@clarinet.com (Brad Templeton) writes:
>> Black holes are really a horse of a different colour, although they have
>> no colour, and no hair.
>Yep. The only things they take with them are mass, charge, and angular
>momentum. :-)
They can perhaps be naked, however:-). Or maybe not. A few physicists are
worried about finding a naked singularity (the prudes!). If anyone ever
does none of the rules are any good anymore.
--
Andy Nelson |Disclaimer:The University of Arizona
an...@neutrino.physics.arizona.edu|only acknowledges my existence when they
an...@neutron.physics.arizona.edu |want money from me. The feeling's mutual:-)
Peter C. McCluskey
|> If you're watching something fall into a black hole, you _never_ see it
|> reach the event horizon. But if you _yourself_ are falling into a black
|> hole, the time it takes to cross the event horizon is, as the article
|> points out, "quite finite." Instead it's the singularity of the black
|> hole which is what takes an infinite amount of time to cross.
How is it possible to know what happens at the singularity? I was under
the impression that GR breaks down at the singularity because the assumption
of local flatness of space-time is violated, and that with GR not working we
don't have any theory to describe what is happening there.
--
----------------------------------------------------------------------
>> Peter McCluskey >> p...@cs.brown.edu >> Reunite Gondwanaland!
----------------------------------------------------------------------
daniel fox
infinite amount of time only to an observer placed an infinite distance from
the black hole. If the observer is a finite distance the object only takes a
finite amount of time.
Try the following thought experiment:
Drop an object into the black hole. A few seconds later drop an observer after
it. Obviously the observer only has a finite amount of time to observe the
object falling into the hole before s/he crosses the event horizon. Obviously
the observer does not overtake the object and cross the event horizon first.
Obviously the object crosses the event horizon first. Obviously the observer
is outside the black hole before the object crosses the event horizon.
Obviously the observer will see the object take a finite amount of time to
cross the event horizon since the observer only has a finite amount of time
until s/he crosses the event horizon him/herself and the object crosses the
event horizon before s/he did.
******************************************************************************
* Daniel B. Fox * All men are mortal. *
* KF9ET * Aristotle is a man. *
* FO...@SILVER.UCS.INDIANA.EDU * Therefore: All men are Aristotle. *
******************************************************************************
* My opinions are my own. Mine you hear me!!! MINE!!! MINE!!! MINE!!!! *
******************************************************************************
Matt McIrvin
>In article <RT0BXB...@west.darkside.com>, m...@west.darkside.com (Erik Max Francis) writes:
>|> If you're watching something fall into a black hole, you _never_ see it
>|> reach the event horizon. But if you _yourself_ are falling into a black
>|> hole, the time it takes to cross the event horizon is, as the article
>|> points out, "quite finite." Instead it's the singularity of the black
>|> hole which is what takes an infinite amount of time to cross.
> How is it possible to know what happens at the singularity?
It isn't, really-- I'm not sure precisely what that sentence meant, but
I would like to point out that in a Schwarzschild hole (the kind in which
the singularity is unavoidable even in an idealized situation) an
infalling astronaut will hit, not just the event horizon, but the
singularity in a finite amount of time. What happens *at* the singularity
is outside the domain of GR, but tidal forces diverge as one approaches
it, so the trip is surely fatal.
Followups set to sci.physics.
Matt McIrvin
>My understanding was that an object dropped into a black hole would take an
>infinite amount of time only to an observer placed an infinite distance from
>the black hole. If the observer is a finite distance the object only takes a
>finite amount of time.
No-- it appears (neglecting the corpuscular nature of light, redshift,
and so forth) to take an infinite amount of time to any observer who
is not also falling into the hole.
[description of thought experiment with an infalling observer following
the falling object-- surely the observer must see the object cross the
event horizon before the observer does]
Actually, the observer sees the object cross the event horizon just as
the observer is crossing the horizon! The two don't collide because
the event horizon is a lightlike surface. The object falls through at
a different place and time, but light emitted from the object stays on
the horizon, and intercepts the observer's eyes as the observer falls
through. The observer consequently never sees the object wink out
of visibility; the moment when the observer sees the object pass through
the horizon, the observer is going through it as well.
The best way to see how all of these things work is to spend some
time studying a spacetime diagram in Kruskal (some say Kruskal-Szekeres)
coordinates. Many textbooks on general relativity contain such diagrams,
and it's possible to understand them without learning the rest of GR.
Wald and Misner et al. both contain them.
Followups set to sci.physics (I think I did it right this time).
Dan'l DanehyOakes
>My understanding was that an object dropped into a black hole would take an
>infinite amount of time only to an observer placed an infinite distance from
>the black hole. If the observer is a finite distance the object only takes a
>finite amount of time.
Yes, but. . .
there is no such *thing* as a "finite distance" from the black hole. A distance
is measured between two defined endpoints, and there *is* no endpoint defined or
definable at the black hole.
Your thought-experiment breaks down. . .
>Try the following thought experiment:
>Drop an object into the black hole. A few seconds later drop an observer after
>it. Obviously the observer only has a finite amount of time to observe the
>object falling into the hole before s/he crosses the event horizon. Obviously
> the observer does not overtake the object and cross the event horizon first.
>Obviously the object crosses the event horizon first. Obviously the observer
>is outside the black hole before the object crosses the event horizon.
>Obviously the observer will see the object take a finite amount of time to
>cross the event horizon since the observer only has a finite amount of time
>until s/he crosses the event horizon him/herself and the object crosses the
>event horizon before s/he did.
. . . because it relies on an observer who is herself dropped into the event
horizon. As time is dilated for the observer drawing nearer the event horizon
(remember, an infinite amount of time is being dilated to a finite amount;
therefore there is infinite time dilation), the concept of "before" and "after"
become, shall we say, problematic.
The effect upon this of a quantized spacetime is left as an exercise for the
mathematically-inclined reader.
People wanting a good science-fictional treatment of this process are invited
to read Fredrik Pohl's GATEWAY and BEYOND THE BLUE EVENT HORIZON, in that order.
It isn't necessary to imagine the world ending in fire
or ice -- there are two other possibilities: one is
paperwork, and the other is nostalgia. When you compute
the length of time between THE EVENT and THE NOSTALGIA
FOR THE EVENT, the span seems to be about A YEAR LESS IN
EACH CYCLE. Eventually within the next quarter of a
century, the nostalgia cycles will be so close together
that people will not be able to take a step without
being nostalgic for the one they just took. At that
point, everythign stops. Death by Nostalgia.
-- Frank Zappa
Dan'l Danehy-Oakes, Net.Roach
My opinions do NOT represent Pacific Bell,
Professional Development, or anyone else.
But I'm willing to share.
Erik Max Francis
> How is it possible to know what happens at the singularity? I was under
> the impression that GR breaks down at the singularity because the assumption
> of local flatness of space-time is violated, and that with GR not working we
> don't have any theory to describe what is happening there.
You're absolutely right. Fortunately for our spaceman falling into the
black hole, he never reaches the singularity (it takes an infinite amount
of time), and thus we don't really have to address what happens when he
does reach it.
John C. Baez
>br...@clarinet.com (Brad Templeton) writes:
>
>> Black holes are really a horse of a different colour, although they have
>> no colour, and no hair.
>
>Yep. The only things they take with them are mass, charge, and angular
>momentum. :-)
I am planning on leaving my mass, charge, and angular momentum to
science.
Matt McIrvin
>You're absolutely right. Fortunately for our spaceman falling into the
>black hole, he never reaches the singularity (it takes an infinite amount
>of time), and thus we don't really have to address what happens when he
>does reach it.
Unless I am mistaken (I got my information from Misner, Thorne, and
Wheeler), the unfortunate spaceman will actually reach the singularity in
a finite time. What happens there is anyone's guess.
Dani Zweig
>>Yep. The only things they take with them are mass, charge, and angular
>>momentum. :-)
>
>I am planning on leaving my mass, charge, and angular momentum to science.
A worthy thought. I guarantee that they will be equitably distributed, in
strict accordance with the law.
RUBIO
>}> effect of gravaty instantaneous or is it also limited by the speed
>}> of light?
Let me answer your question with the results of an experiment
(I'm not sure if it has been actually been performed... any.body?... but
these is what GR says will happen):
Point vector A to the center of where you SEE the sun (that is,
the direction of the incoming light that left ~ 8.5 min ago )
Point vector B in the direction to where the gravitational force
of the sun points.
Question: Are these two vectors parallel?
Answer: No. B points in the
direction that A will be pointing in ~ 8.5 min (assuming only gravitational
forces are at work here so the sun _will_ be there in ~ 8.5 min.).
Note that the above does NOT violate causality, but I'll let you worry about
that.
-jose a. rubio-
Disclaimer: My oppinion should be that of my employer, but it ain't.
Flames are welcome but I flame back.
Bruce Bowen
> Drop an object into the black hole. A few seconds later drop an observer after
> it. Obviously the observer only has a finite amount of time to observe the
> object falling into the hole before s/he crosses the event horizon. Obviously
> the observer does not overtake the object and cross the event horizon first.
> Obviously the object crosses the event horizon first. Obviously the observer
> is outside the black hole before the object crosses the event horizon.
> Obviously the observer will see the object take a finite amount of time to
> cross the event horizon since the observer only has a finite amount of time
> until s/he crosses the event horizon him/herself and the object crosses the
> event horizon before s/he did.
Your conclusion is incorrect. The observer does see the initial object fall
through the event horizon in a finite amount of observer proper time, but the
observer sees it just has he falls thru the horizon. Think of it as the light/
information at the horizon is standing still, and the observer falls through it
at the speed of light.
-Bruce
John C. Baez
Thanks. As long as they are conserved for future generations I will be happy.
Matt McIrvin
>>}> This reminds me of a question I came up with a while ago. Is the
>>}> effect of gravaty instantaneous or is it also limited by the speed
>>}> of light?
> Let me answer your question with the results of an experiment
>(I'm not sure if it has been actually been performed... any.body?... but
>these is what GR says will happen):
[Gravitational acceleration is toward the sun's current position, not
toward the place where it was eight minutes ago]
According to Tom van Flandern, the experiment is performed all the
time. He did numerical modeling which showed the solar system to be
wildly unstable if the gravitational force points toward the sun's
retarded position, or even toward the sun's retarded position
extrapolated along a constant-velocity straight line. Instead, what
you need is what GR predicts: the force points to a position
extrapolated along a spacetime geodesic from the sun's retarded
position. This is (for all practical purposes) the same as the
sun's current position, since the sun is in free fall.
So our continued survival is really a nice test of this statement.
As you say, this apparently "instantaneous" effect happens without
any nonlocal or noncausal process occurring. A gravitational
wave would still travel at c, according to GR; and if the sun
were suddenly to develop a huge quadrupole moment in some violent
nongravitational event, we wouldn't feel the results for eight
minutes.
Steve Carlip
>[...]
>I would like to point out that in a Schwarzschild hole (the kind in which
>the singularity is unavoidable even in an idealized situation) an
>infalling astronaut will hit, not just the event horizon, but the
>singularity in a finite amount of time. What happens *at* the singularity
>is outside the domain of GR, but tidal forces diverge as one approaches
>it, so the trip is surely fatal.
>
rotating black hole (perturbed Kerr solution). There's a nice
article by Amos Ori in Physical Review Letters 68 (1992) 2117, who
presents evidence that the tidal forces are finite and bounded,
and that "a physical object can survive all the way up to the
singularity." This is a fairly new result, and I'm not sure how
carefully it's been checked, but it does make life interesting!
By the way, on the subject of whether an external observer can ever
see a black hole form, I strongly recommend the beginning of chapter
33 of Misner, Thorne and Wheeler, which contains a very nice (and
nontechnical) discussion of this issue.
Steve Carlip
car...@dirac.ucdavis.edu
John F Blanton
rear-end collision problem. Below is a rude diagram of the event:
____ ____
__/ |__/ | <---
|________|________|
O O O O
A B
Car A has mass m1.
Car B has mass m2.
Let's make it easy on ourselves and assume m1 = m2.
Car B has velocity v, it is speeding to the left.
First example, not a realistic scenario:
Perfectly rigid bodies, the automobile bodies and frames are
completely incompressible. Also assume an "elastic" collision.
Elastic is to be defined later if necessary.
At the collision a force is generated between A and B. This force is
F. The force is present only while A and B are in contact; there is
no action at a distance in this case. When B touches A, there is a
force. When B is not touching A, the force B exerts on A and the
force A exerts on B disappears. Since the collision is elastic, A and
be do not remain in contact with each other forever, so the force
between them exists for only a finite period of time. Call this
period of time dt. Make it easy on ourselves again and assume that
the force F is constant during the time interval dt. The problem
works out the same if F is not constant during dt, but the math is
needlessly complicated.
By the condition of the incompressible bodies, I can say that dt is
zero, or nearly so. This is because: 1) the two cars touch (hit), 2)
since A is not moving, and B is, for a time their centers get closer
together. D is the distance between centers when the cars touch
initailly, and d is the distance at other times. When d < D, the cars
are in contact (touching), and the bodies are compressed. Since the
bodies are "incompressible" this time dt that the bodies are
compressed must be vanishingly small. So let's say dt --> 0
(approaches zero as a limit).
The above was a simple minded argument for saying that dt --> 0. If
you will accept that dt --> 0 for highly incompressible bodies, you
can ignore the argument.
Since dt is finite, the cars must have bounced apart. From the
geometry of the situation, this can only happen if A speeds up (to the
left) or B slows down or both. From the principle of conservation of
linear momentum, the momentum of the configuration is the same before
and after the collision.
Before: p = m v + m v
A-before B-before
= 0 + m v (A was not moving).
B-before
After: p = m v + m v
A-after B-after
Also the energy is conserved (perfectly elastic collision).
2 2
Before: E = 1/2 m (v ) + 1/2 m (v )
A-before B-before
2
= 1/2 m (v ) (A was not moving)
B-before
2 2
After: E = 1/2 m (v ) + 1/2 m (v )
A-after B-after
(we have yet to determine v )
A-after
The above two equations have two common solutions:
1) v = 0,
B-after
2) v = v
B-after B-before
Solution 2) implies v = 0. Can't be, because of the geometry.
A-after
That would mean B went right through A (some automobile collisions
give this appearance). Let's reject 2).
Equal car masses and elastic collision result in solution 1). Result
1) is not necessarily true for other conditions.
Ta-daa! A has accelerated. How long did it take? Zero time
(practically). What was the acceleration? Infinite acceleration from
the relation
a = dv/dt
where a is the acceleration, dv is the change in velocity, and all of
the other terms are as defined previously. Since dv is not zero and
dt --> 0, a must --> infinity.
What is the force, F? From the 2nd law
F = m a
F --> infinity.
What are the forces on A?
F is the down gravity force.
g
F is the upward pavement force.
p
F is the sideways frictional force of the tires on the pavement
T
F is this (practically) infinite force of impact.
F and F cancel out and can be ignored, except with Coulomb friction
g p
they determine maximum F .
T
F << F (F is vanishingly small compared to F).
T T
Car A takes off at velocity v regardless whether the brakes
B-before
were locked or not. THIS ONLY HAPPENS WITH PERFECTLY RIGID BODIES,
BRAKE MECHANISMS, TIRES AND EVERYTHING ELSE. A similar event occurs
when m does not equal m , but there is not a 1:1 transfer of
A B
velocity.
--------
Second example, non-rigid bodies, more realistic:
Forget all of the math, assume the cars are made of rubber. B hits A,
they are in contace for a MUCH LONGER period of time, the force of
contact is not only finite, but it is less than F . The collision can
T still be perfectly elastic, but A can stay rooted to the ground,
absorb the impact of B and bounce car B backwards at its original
speed. Of course car A deforms a lot during this process (as does car
B) but only if v is large (a relative term). In a real
B-before example, both cars have springy bumpers, the speed of impact
is about 1 mph, car A locks its brakes, and car B bounces off.
Passengers of both cars get rattled, but car A doesn't take off into
the intersection.
I have gone on for over 150 lines with this grade-school level
explanation, so I'm going to stop. Additional questions are welcome,
but I may be slow in answering.
John Blanton
bla...@lobby.ti.com (my most reliable e-mail address)
Emory F. Bunn
[In answer to the question of whether the gravitaional force from the
Sun on the Earth points towards the sun's present position, or towards
some "retarded position": In order for the solar system to behave
as it does,]
> What
>you need is what GR predicts: the force points to a position
>extrapolated along a spacetime geodesic from the sun's retarded
>position. This is (for all practical purposes) the same as the
>sun's current position, since the sun is in free fall.
>
>So our continued survival is really a nice test of this statement.
>
>As you say, this apparently "instantaneous" effect happens without
>any nonlocal or noncausal process occurring.
This is a nifty result. It becomes somewhat less surprising if you
think back to your special relativity and E and M. Remember that if
you have a charge whizzing by you at constant velocity (that is, along
a geodesic), and you measure the electric field caused by that charge,
it always points towards the "present position" of the charge (That
is, its position at time t=now as measured in your inertial reference
frame). This occurs in spite of the fact that electromagnetic effects
propagate at speed 1 (or c if you prefer), so the electric field you
feel now was actually caused when the charge was at some retarded position.
I take it that the gravitational effect Matt describes is analogous.
-Ted
Erik Max Francis
> As two others have pointed out this is wrong. To be a little more specific,
> the spaceman reaches the singularity in finite _proper time_, that is time
> integrated along his path (the time interval measured by his wrist watch,
> say). In fact, for a black hole of mass 10^6 solar masses (large enough
> that tidal effects at the event horizon are negligible, and the spaceman
> would not be torn apart by them), the proper time from event horizon to
> singularity is something like 10^-5 seconds. Discussion of what the
> spaceman sees while on his way to the singularity is somewhat academic,
> shall we say.
Quite correct; sorry about the confusion. I was 1. misinformed by a
friend and 2. misread an article concerning black holes and general
relativity. Oops.
Thanks for pointing out my mistakes.
Gregory E. Garland
....[snip]
|>
|> Car A takes off at velocity v regardless whether the brakes
|> B-before
|>
|> were locked or not. THIS ONLY HAPPENS WITH PERFECTLY RIGID BODIES,
|> BRAKE MECHANISMS, TIRES AND EVERYTHING ELSE. A similar event occurs
|> when m does not equal m , but there is not a 1:1 transfer of
|> A B
|>
|> velocity.
While your analysis is basically okay, it ignores the fact that cars
are _designed_ to crumple up during a collision, expending the force
of the collision on deforming the car body while trying to keep the
passenger compartment intact.
I had assumed you intended to show that with some ball park estimates
for the force exerted on the car during the crash (perhaps taken from
real crash experiments) and the frictional force between the road and
car that the tires skid instead of rolling even if the brakes are off.
While that still may be true, the assumptions you have made in your
model are too far from the real world to prove it.
|>
|> --------
|> Second example, non-rigid bodies, more realistic:
|>
|> Forget all of the math, [...snip...]
WHAT?! NEVER! :-)
--
Greg Garland - Alive, occupying space, and exerting gravitational force
MS 62-024, Harris Semiconductor Sector, PO Bx 883,
Melbourne FL 32905. g...@beep.mis.semi.harris.com
"Never let the facts interfere with your perception of reality."
Blair P. Houghton
>They can perhaps be naked, however:-). Or maybe not. A few physicists are
>worried about finding a naked singularity (the prudes!). If anyone ever
>does none of the rules are any good anymore.
If anyone does see one would they please scan it and post
the GIF to alt.binaries.physics.erotica?
--Blair
"Probably moderated, that."
Michael Friedman
>Okay, I'll be stupid now! :-)
>We have all (well almost all) agreed that the original question was
>silly because there is no such thing as an incompressible substance.
>But is that true?
Yes.
>When a star colapses and becomes a black hole, what
>is the density of the material?
Pretty damned high. In fact, words like "density" may no longer apply
because they assume that length measurements are applicable.
>Isn't this theorectically a mass of
>nuclei smushed up against ecah other?
No. That would be a black dwarf, an object that is much less dense
than a black hole.
>If so, how would one compress
>this any further?
A black dwarf? By turning it into a neutron star or a black hole.
A black hole? Not clear if you can.
>Of course, it is probably meaningless to think
>about applying a force to such an object...
Gerry, if you throw a baseball at it what do you think happens? You
just applied force.
What you can't do is to push it or have it push other things. If you
throw things at it to move it then you will see the even horizon
deform due to acceleration. I think the deformation will propagate at
the speed of light, but it might be slower.
--
-------------------------------------------------------------------------------
I am not an official Oracle spokesman. I speak for myself and no one else.
Matt McIrvin
[about the non-retarded nature of the gravitational force in celestial
mechanics]
>This is a nifty result. It becomes somewhat less surprising if you
>think back to your special relativity and E and M. Remember that if
>you have a charge whizzing by you at constant velocity (that is, along
>a geodesic), and you measure the electric field caused by that charge,
>it always points towards the "present position" of the charge (That
>is, its position at time t=now as measured in your inertial reference
>frame). This occurs in spite of the fact that electromagnetic effects
>propagate at speed 1 (or c if you prefer), so the electric field you
>feel now was actually caused when the charge was at some retarded position.
>I take it that the gravitational effect Matt describes is analogous.
Yes, except that now the geodesics are general-relativistic ones rather
than special-relativistic ones, so the massive object doesn't have to
be moving in a straight line, just in free fall.
Blair P. Houghton
>[Contract] A black hole? Not clear if you can.
You can't, actually, since its (Schwartzchild) radius is
determined by its mass; no matter how small you make the
actual blob of matter, the "black hole" has a fixed size,
and therefore a fixed "density."
--Blair
"I love contradicting myself."
Mark E. Horning (Captain Neutrino)
>In article <1993Jan16....@mksol.dseg.ti.com>, bla...@mksol.dseg.ti.com (John F Blanton) writes:
>
>
>....[snip]
>|>
>|> Car A takes off at velocity v regardless whether the brakes
>|> B-before
>|>
>|> were locked or not. THIS ONLY HAPPENS WITH PERFECTLY RIGID BODIES,
>|> BRAKE MECHANISMS, TIRES AND EVERYTHING ELSE. A similar event occurs
>|> when m does not equal m , but there is not a 1:1 transfer of
>|> A B
>|>
>|> velocity.
>
>
>While your analysis is basically okay, it ignores the fact that cars
>are _designed_ to crumple up during a collision, expending the force
>of the collision on deforming the car body while trying to keep the
>passenger compartment intact.
>
ford could punch through a brand new vehicle like a knive through
butter, and drive off afterwards.
>I had assumed you intended to show that with some ball park estimates
>for the force exerted on the car during the crash (perhaps taken from
>real crash experiments) and the frictional force between the road and
>car that the tires skid instead of rolling even if the brakes are off.
>While that still may be true, the assumptions you have made in your
>model are too far from the real world to prove it.
>
>
>|>
>|> --------
>|> Second example, non-rigid bodies, more realistic:
>|>
>|> Forget all of the math, [...snip...]
>
>WHAT?! NEVER! :-)
>
>
dirac deltas and Laplace transforms.
Mark E. Horning, Physics Back off man, I'm a scientist.
mhor...@pan.calpoly.edu Bill Murray, Ghostbusters
mhor...@data.acs.calpoly.edu
A penny saved is a congessional oversite.
>
John F Blanton
automobile collision. It really glosses over a bunch of stuff and
treats ideal cases.
However, I think he indicates I stated the front car's tires will
slide whether the brakes are set or not. It was not my intention
to state this outright, however, in the case of a PERFECTLY rigid
body (frame, sheet metal, tire rubber, but the wheels can still
roll), the tires will skid for a moment after impact, even if the
brakes are not set, then they will start rolling without
skidding after the tangential frictional force spins them up
fast enough.
This (perhaps unexpected) result can still be achieved even in
actual collisions with real cars, if the accelerations are high
enough.
A more realistic situation analysis is forthcoming if you want,
but it will take a while. I have to go work out the details
on paper. I can't just type it in like the simplified example
I posted previously.
Forget the math? Who me? Just another way of my saying that
it's getting tiresome typing all of this in. Math is a lot more
natural with pen and paper. There's gotta be a better way..
Ross Smith
I suggest you leave your baryon number too. Otherwise you may find yourself
strongly forced to comply with some conservative laws.
--
...... Ross Smith (Wanganui, NZ) ...... al...@acheron.amigans.gen.nz ......
"I blame you for the moonlit sky and the dream that died with the Eagle's flight
I blame you for the moonlit nights when I wonder why are the seas still dry
Don't blame me, sleeping satellite" (Tasmin Archer)
--