View Full Version : More "weird gravity"

publius

2006-Mar-26, 01:20 AM

The recent threads about the gravitomagnetic fields had me Googling and I stumbled across this:

http://www.physorg.com/news10789.html

Paraphrasing, a Dr. Franklin Felber claims that he has an exact solution of Einstein's field equation for masses moving at high v/c, and claims an "antigravity" effect when a mass reaches a velocity of "57.7%" c. He claims there will be a gravitational repulsion in a "narrow beam" in front of the mass, and that repulsion will increase as v increases.

I want to see the results of this claim.

-Richard

EDIT: Dr. Felber's paper on this: http://arxiv.org/PS_cache/gr-qc/pdf/0505/0505099.pdf.

Looks very interesting to me. He considers a black hole approaching a small "payload" mass and calculates how the payloads behaves from the frame of a distant observer. The black hole appears to push the payload away from it as it approaches. He also says there are "cosmological implications" to this, which I'm sure the astronomers here will appreciate far better than I.

publius

2006-Mar-26, 01:36 AM

Rather than editing again, I'll just post a reply. I knew there was something familiar about "57.7%". That is 1/sqrt(3). I'm an electrical power nut who has been described by friend as "daydreaming 3-phase" all day long. Well, basically all you need to know about 3-phase is the square root of three which pops up everywhere. <rofl>

And here is it in this crazy anti-gravity (well gravitational repulsion) stuff. When the incident black hole's approach speed is c/sqrt(3), it starts pushing the payload away (coming at it dead on).

-Richard

01101001

2006-Mar-26, 03:21 AM

The recent threads about the gravitomagnetic fields had me Googling and I stumbled across this:

http://www.physorg.com/news10789.html

If you read Is this possible? (http://www.bautforum.com/showthread.php?t=39024) in Questions and Answers, you could have stumbled across it too.

hhEb09'1

2006-Mar-26, 03:34 AM

Rather than editing again, I'll just post a reply. I knew there was something familiar about "57.7%". That is 1/sqrt(3). I'm an electrical power nut who has been described by friend as "daydreaming 3-phase" all day long. Well, basically all you need to know about 3-phase is the square root of three which pops up I thought it looked familiar too :)

It's also the Euler-Mascheroni constant (http://mathworld.wolfram.com/Euler-MascheroniConstant.html), we've talked about it before some (http://www.bautforum.com/showthread.php?p=187611#post187611).

publius

2006-Mar-26, 03:53 AM

I need to improve my BAUT forum searching skills. I did a title search for this Dr. Felber as well as 'gravity' and "antigravity" but didn't find anything. I should have done a full text search. <sigh>

I also stumbled across a criticism that he didn't try to rule out his repulsion effect was one of those "coordinate artifacts" that can trip you up rather than a real physical effect.

-Richard

hhEb09'1

2006-Mar-26, 03:57 AM

No big deal, it just means that you've been ToSeeked (http://www.bautforum.com/showthread.php?p=256104#post256104)! :)

John Dlugosz

2006-Mar-29, 07:24 PM

How does that work within relativity? The velocity is relative to some reference frame. How the object be emitting a beam "forward" in one reference frame and not in another?

publius

2006-Mar-30, 06:35 AM

John,

The "beam" analogy can be sort of confusing, and is probably a poor one. What is being claimed is that in a narrow region in front of the approaching object, gravity appears to be repulsive in certain frames of reference when the object reaches the "magic velocity".

Imagine the test mass sitting still in a frame of reference. A massive object comes flying in at >.577c, on a collision course with the test mass. In our frame the object the test mass will appear to accelerate away from the approaching object. The approaching object will eventually catch up and hit the test mass in our frame, however. It will just appear to "run away" from it, rather than accelerating toward it.

As understand what the author is claiming, the effect occurs only in very narrow angle about the head-on path.

-Richard

Ken G

2006-Mar-30, 07:15 AM

The flaw I see in this argument is that if the speed were exactly .577... times c, then apparently there is a transition from attraction to repulsion, meaning that there could be no gravity at all at the cutoff speed. That is highly implausible of course, as an approaching black hole at exactly that speed would therefore never influence the test particle at all. That also implies that a test particle dropped toward a black hole at exactly that speed would neither speed up nor slow down, yet this must be part of the standard Schwarzchild solution. It is not plausible that this is really a new class of solution, though of course I have no idea the mathematics of it and maybe it is just some kind of artifact rather than a true error.

publius

2006-Mar-30, 07:33 PM

Ken,

That's the way it seems at first blush, but this is just us thinking in terms of Newtonian-acting gravity that we are familiar with.

Consider watching a test mass with zero L fall into a black hole from a reference stationary to the black hole. In Newtonian terms, this is a just a straight-line acclerated trajectory toward the center of the black hole.

From our POV, according to GR, we never see the test mass cross the event horizon -- its clock stops and its light becomes "infinitely red shifted" and it appears to "freeze" at the event horizon.

For that to be true, and this should then be in the Schwarzchild metric, then at some point, the test mass will have to appear to slow down, and that means the force we see on it would have to become repulsive at some point.

From the "Felber effect", does this mean that this would occur when the accelerated test mass reaches 0.57c in our frame?

Now, consider the black hole to be moving in our frame. Because of the freezing at the even horizon, the test mass will appear to "stick" to the event horizon and therefore reach a final velocity equal to the black hole's velocity. So (as long as the test mass does not have enough velocity to escape the black hole), no matter what trajectory it in intially follows, it must end up stuck to the event horizon and dragged along with the black hole in our frame.

Felber's paper from that link has a graph of test mass velocities vs distance to the black hole. You'll see all paths, even the intially "attractive" paths, converge to a final value equal to the black hole's velocity.

The trajectory for the magic velocity does indeed appear to be flat until the black hole gets very near, at which point it begins to push it *away*, and reach a final velocity equal to the incident velocity, where it has "stuck" on the event horizon.

This sticking at the even horizon is what makes this very different from our Newtonian intuition.

-Richard

dgavin

2006-Mar-30, 08:38 PM

I read the article and there are some definate issues with what he's proposing.

First, -if- gravity trualy became anit-gravity as .577c in a narrow beam, this would be require fundament change in particle physics. It means that either 1. Both accelataion and mass cause gravity/anit-gravity effects, or 2. that the Graviton/Repuslaton/Dilaton theory that was discarded some decades ago was correct.

My -guess- is that as the math hasn't been done before, the result is ethier an artifact from GR not having been tested and adjust in that direction, or an artifact of bad math.

Ken G

2006-Mar-31, 08:49 AM

For that to be true, and this should then be in the Schwarzchild metric, then at some point, the test mass will have to appear to slow down, and that means the force we see on it would have to become repulsive at some point.

This is an interesting point, and it all gets quite confusing because there are multiple coordinate systems one might use to describe what is happening, and the words can come out sounding quite different too. Since I've never done a calculation like this, I'm hampered by not knowing the details. But I would tend to think that if you want to use a coordinate system that includes extreme time dilation near the event horizon, then you must also include extreme length contraction. If so, it is not necessary for the object's speed to slow down, as it is facing more and more distance to travel relative to its own length, even as its clock slows down. Thus its speed can continuously increase and never reach the event horizon (everything I've said is of course in our own reference frame back here on Earth, we all know that there is no change in the clock from the point of view of the object going across the event horizon, and in that frame it happens in a finite time). In short, if the object is a ruler with a clock stuck to it, and if we measure its speed by the inverse of the time it must register on the clock to travel its length, then I don't think (though I'm not sure) this concept of speed will ever decrease. (It's what one might call "proper speed", the ratio of proper distance to proper time). So it now seems likely to me that the "Felber effect" is a coordinate effect, like the coriolis or centrifugal effect. It appears in a certain coordinate choice, but is not physically seperable from the coordinates. I would look carefully at the coordinates he is using, and see what happens in other coordinates. I'll bet the effect goes away.

Felber's paper from that link has a graph of test mass velocities vs distance to the black hole. You'll see all paths, even the intially "attractive" paths, converge to a final value equal to the black hole's velocity.

Yeah, that might require kind of a bizarre concept of distance, that is extrapolated from our concept into the vicinity of the black hole. Coordinate systems in GR are a real tangle, I can't make much sense of it, and tend to suspect that the only truly physical statements have to be expressed in coordinate invariant forms. I'm suspecting that the .577c result is not coordinate invariant, so I'm highly skeptical of any physical significance of this number.

publius

2006-Apr-01, 02:56 AM

Ken,

I was looking around and found this:

http://www.fourmilab.ch/gravitation/orbits/

This has a java applet that (purports, but I assume it is correct) to show the orbit of a test mass very near a black hole in strongly curved space-time. This is based in the Schwarzchild metric.

You'll see it gives an expression and plot for the gravitational potential, and it is has a noticable non-Newtonian form. It has a local minimum at some radius, then starts to increase to a peak (which if I understood correctly, occurs right at the Schwarzchild radius), then drops off towards -infinity. If something moves to the left of that peak, it's gone. The orbit plotted gets very near that peak.

Now, interpreting force as the slope of the potential (and I know in relativity, and certainly GR, the notion of force gets tricky), then that would agree with gravity becoming repulsive near the event horizon.

However, looking a the orbit, the test mass appears to increase in speed even when it is going up the potential hill. And I don't know what speed definition is being used, according to your post, either.

Note the mass moves around and around the black hole, but doesn't follow anything like a Newtonian orbit. I don't know if the trajectory is a closed path or not. It is bounded, but goes all over the place.

-Richard

publius

2006-Apr-01, 03:30 AM

Ken,

Arrghh -- it's been so long I've forgotten a lot. The potential plotted is the "effective potential", not the regular gravitational potential.

If I remember my mechanics right, this comes from a switch to polar coordinates. The theta coordinate just depends on L -- that is d(theta)/dt is just something that depends on L. For the r coordinate, you get an equation for the r motion, that is something to give r(t), and this effective potential business comes from that, and includes a "centrifugal force" term from L.

For an attractive inverse-square force, that effective potential will always have a local minimum. Coming in from the right at infinite r, it decreases from zero (here the normal gravitational term dominates), then reaches a minimum, the turns up towards +infinity at r = 0. That upturn is due the "centrifugal force".

The difference with the Schwarzchild potential is that turns back down again. If a particle gets past that peak, it goes past the event horizon and never comes out.

So there is no repulsive gravitational force shown here at all.

-Richard

Ken G

2006-Apr-01, 03:16 PM

Yes, I think you are right about the polar coordinates, and the effects of conserving angular momentum which looks like a repulsive 1/r^3 potential in polar coordinates. But the coordinates of those simulations are as valid as any, so I think it's fine to say you have repulsion in those coordinates. It is very easy to get caught up in specific aspects of a particular coordinate system, and hard to see the aspects that are invariant to coordinates. I think that might be Felber's mistake, but I'm just guessing. I only know that you can't say much without really doing the calculation, so I'll stop there.

publiusr

2006-Apr-06, 06:36 PM

Arrghh -- it's been so long I've forgotten a lot. The potential plotted is the "effective potential", not the regular gravitational potential.

So there is no repulsive gravitational force shown here at all.

-Richard

Spaceflight is never easy.

tarajee

2006-Apr-11, 07:04 PM

The final theory.

Writer of the book “The final theory,” has done a fine work where as he speaks for failure of the theory of gravity already presented by the old thinkers. But one thing is strictly objectionable to use the hard language for the scientists gone from the field, above all when he himself has failed to produce workable theory. He has given a theory, a dull theory but in the appealing words. You are right when you say how nicely he has presented the totally wrong theory. Up to this time only workable theory is from Newton.

The author of the book has presented a theory relative to the expansion of the structure of the atom. So every thing is bound to expand. If an apple is falling down towards the earth, actually it is not in falling process but it is expanding while at the same time earth is also becoming big. This process of expansion minimises the gap between the apple and earth resulting in a crash of both the things. In such a case the atoms of every sky object will expand and so sun moon and earth with other planets will try to fill the gape between each other. What will happen….a crash, but things are not going to obey the writer. Where I may place the intellectual power of the persons who say that writer has done a remark able work while he has come in this field with a remark able blunder? The only thing which goes in the favour of the writer is that he knows how to write. Final theory is still far far away from the minds of the people. Tarajee

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