Posted on 11/28/2003 1:49:27 PM PST by LibWhacker
We've had threads on that:
First speed of gravity measurement revealed .
Berkeley Lab Physicist Challenges Speed of Gravity Claim.
No. There is a kook named van Flandern who maintains that gravity propgates at (almost) infinite speed. In the classical physics domain, one can demonstrate--as Eddington did in 1905--that gravity must travel at infinite speed.
The reason is a sort of inverse of the "Poynting-Robeson" (spelling?) effect, which causes rays of light falling on the earth to appear to come at a slight angle (due to the motion of earth relative to the Sun). Similar abberations can be seen in starlight. Unlike light, the analog of the P-R effect in gravity tends to accelerate the planet (the radial vectors point in opposite directions; the light pressure tends to retard the earth's motion; gravity (if travelling at finite speed) ought to accelerate it. It can be shown that in a very short time--a few thousand years--all the planets should be flung from the Sun if gravity propagates at finite speed.
However.
I corresponded with Professor Carlip of U.C. Davis upon discovering van Flandern's assertions, and as Patrick Henry's article link above indicates, under general relativity it can be conclusively shown that gravity waves are radiated which precisely cancel out the supposed acceleration which appears in classical mechanics.
--Boris
I have been thinking about this post for the past couple of days.
The density of the interstellar medium is pretty low. Not only is it made up of primarily hydrogen and helium (close to 99 percent); it has and average density of only about one atom/molecule per cubic centimeter. Contrast that with our atmosphere, at the earth surface, which contains in the neighborhood of 1 x 1020 molecules per cubic centimeter.
Using only hydrogen and helium, could a machine replicate itself? Think of the complexity of the machine and the requirement to gather the correct material and mass for replication. For instance, could a machine replicate itself using only our own atmosphere as a source of material?
So if the ISM cannot provide the required material for replication, you must then use a solar system for this effort.
Again I ask, what would produce the delta V required to escape the gravity well of the star? Even as far out as Jupiter, the escape velocity from our own Sun is about 16 kilometers per second. And if the probe were zipping thru a solar system fast enough to already escape the stars gravity well, could it gather enough of the “right stuff” to replicate itself?
Again I ask, what would produce the delta V required to escape the gravity well of the star? Even as far out as Jupiter, the escape velocity from our own Sun is about 16 kilometers per second. And if the probe were zipping thru a solar system fast enough to already escape the stars gravity well, could it gather enough of the “right stuff” to replicate itself?"
I take it as a given that the probe must use 'local matter' to build a copy of itself. That is one reason some suns will be 'dry wells' (if they have no planets or rocks).
Again, I see no reason why a 10,000 or 100,000-year-old technological civilization would find the delta-vee particularly daunting. Remember we posit that they can reach 0.05C (maybe a Bussard ramjet). On the way out, they can afford to take their time. A solar sail. A solar-electric thruster. Other technologies we do not know about yet. But I am certain they could find a way to leave one target system for the next.
--Boris
The rub is you still cannot violate physics no matter how advanced. I assume you need to get all the "right material" for whatever technology you need to escape the gravity well of the star. You also need to find the right "stuff" to replicate your machine. This adds the complexity of either building it from materials found in asteroid belt/cometary clouds or requiring the added delta V to get out of a planetary gravity well also.
Last I heard, there were comets and meteors being detected a good piece of the distance from here to Alpha Proxima. Or is it just comets out that far?
At any rate, How did Voyager escape the Solar System? If I had all the time in the world, why wouldn't solar sails, bootstrapping the big planets, and quite a modest hydrogen jet take me out of the solar system? For that matter, why can't I hitch rides on a comet with a highly eccentric orbit?
And if the probe were zipping thru a solar system fast enough to already escape the stars gravity well, could it gather enough of the “right stuff” to replicate itself?
It could if it made prior vector-matching pick-up arrangements with the "stuck" mining probes that fell permanently into the gravity well.
You got lots of rubs. If the posited intelligent ET's have nanotechnology, all they need is "rocks". I.e., a source of metals, silicon, possibly water. Recall that Drexler posited a box into which one stuffs dirt or grass, air, and water...and steaks emerge at the other end. "After all, that's what a cow does," he said, or something similar...
--Boris
From boris: "On the way out, they can afford to take their time. A solar sail. A solar-electric thruster. Other technologies we do not know about yet. But I am certain they could find a way to leave one target system for the next."
Even with my limited grasp of physics, I know you can't have it both ways. It either does or does not require sufficient acceleration to escape gravitational attractions. I learned that sufficient acceleration is required...which is why STS is strapped to the back of a monster, kick-a$$ rocket. Otherwise, my Dad in his little Cessna (if he had adequate oxygen, etc. for himself and his airplane motor) could casually fly upwards until he reached the Moon. Might take him 5 years, but what's the hurry?
Personally, I think you've watched Star Trek: The Motion Picture once too often, boris.
You are wrong. "Escape velocity" is merely the minimum velocity that one can apply instantaneously (or nearly so) to escape a gravity field. A Cessna obviously is a poor choice of vehicle for use in a vacuum.
But a "skyhook" or "orbital elevator" could indeed operate at 5 mph if it wanted to...all the way to geosynchronous orbit (22,300 miles up). From there, you can deploy a light sail and lazily spiral out at, say 0.001 "G" acceleration. Eventually you will have enough "delta-vee" to escape the Sun's gravity well.
Period.
--Boris
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