on the question of "What is the speed of gravity?"

[buckminster]

In the excellent article:

http://www.straightdope.com/mailbag/mgravityspeed.html


I guess I'm just not hip to all the new-fangled quantum gravity stuff. As I recall Old School Relativity sez gravity is a property of space-time in response to the mass of an object and so there was nothing to propagate.

There's a lengthy description as to why gravity must act at the speed of light cuz nothing in this universe happens instantly.
Hmmmm,

OK, so what are your thoughts on superluminal signaling, entanglement and the other instantaneous action at a distance
quantum effects? Or are you gonna take the easy way out and just call them "spooky" like Einstein.

Well, you may have stepped in the proverbial bucket of shit on this one. :smack:

Don't ya just love quantum craziness?!

best regards,

buck
____________________________________________________
As Ricky Ricardo used to say, "Lucy, you got a lot of explaining to do." - Paul Graham

[C K Dexter Haven]

Gravity is almost but not quite instantateous. For instance, if we're in a conversation group and everyone is laughing and having fun, and then someone says something like, "Sorry to change the subject, but I wanted you to know that I've been diagnosed with terminal cancer", man, gravity takes over real fast.

[Nametag]

Sigh. Matter moves, Bucky. A sudden redistribution of mass leads to a sudden redistribution of gravity (sorry, saying that badly), which does indeed need time to propagate. That's an immediate consequence of "old-school relativity," and it shouldn't have taken you by surprise.

"Superluminal signaling, entanglement and the other instantaneous action at a distance quantum effects" do not, in fact, require anything to propagate faster than the speed of light -- it just seems that way because researchers have been presenting their results in the most sensational way possible, while misrepresenting the nature of the "speed of light" limit.

[ronbo]

For what its worth, I think that The Great and All Knowing Cecil gets something wrong in this response.

First of all, there's no such thing as "instantaneous" transmission of anything.

I believe you will find that in Quantum Mechanics, information can pass instantaneously.

[eburacum45]

Yes, but no useful information can be sent faster than light.
(unless that has changed since 1993, which is the date of this Wikipedia reference (http://www.wikipedia.org/wiki/Quantum_teleportation)

[buckminster]

Originally posted by Nametag
Sigh. Matter moves, Bucky. A sudden redistribution of mass leads to a sudden redistribution of gravity (sorry, saying that badly), which does indeed need time to propagate. That's an immediate consequence of "old-school relativity," and it shouldn't have taken you by surprise.


Actually anything to do with quantum behavior surprise the hell outta me. That sentiment was expressed by this wild haired guy I talked with while sitting on the grass in front of the UCEN during my last semester in college. He sure seemed like a pretty smart dude, a guy by the name of Richard Feynman. Funny as hell, too.

I recall this cool 1950's B/W physics movie with a big rubber sheet with three balls of varying size and weight sitting on it. The big ball started moving and the depression it created in the sheet also moved. As it approached one of the other balls, that ball started moving as did the indentation it created. Guess what happened? As if by magic, the two balls collided and the resulting indentation was even deeper than first ball. Moving mass, gravity wells, no propagation delay because gravity was not considered the same kind of field as an electric or magnetic field.

As far as I know none of the experiments intended to detect gravity waves has yielded definitive results yet. But then I'm old and clueless...

best regards,

buck

[C K Dexter Haven]

<< For what its worth, I think that The Great and All Knowing Cecil gets something wrong in this response. >>

Just to note, ronbo, that this Staff Report (like the other Staff Reports) is not written by Cecil, but by Staff -- in this case, Chronos, who's finishing up his Ph.D. in astronomy and knows whereof his speaketh. Cecil only writes his weekly column; Straight Dope Staff write about two Staff Reports a week. Cecil doesn't get anything wrong, but Staff sometimes do.

[chunda21]

This one REALLY interests me, although I'm only a dilletante on this subject (my degree is in Theology - although I DID use quantum mechanics as part of an argument on the probable existence of a Deity of some kind ...)
My question is this - since we are biologically designed to perceive electromagnetic wavicles (light), but not electrogravitational or magnetogravitational waves (unified field theory moots the existence of such), how is it possible to measure it? If you can't measure something, it is not science, it is opinion.
Electromagnetic waves travel at c (sorry for stating the obvious). But, as we are not built to perceive these other, possibly non-existent "waves", I would have thought it impossible to design anything to calculate their rate of propagation. We would not know how to detect it, as all experimental equipment is anthropomorphic - it parallels our senses in some way. Although I would agree that "instantaneous" transmission is highly unlikely if not impossible, there is no way of determining whether or not gravitational waves travel faster than, slower than, or at the same velocity as, electromagnetic waves.

[sweeney03]

Here's a good link:

http://www.missouri.edu/~news/releases/decjan03/gravitymeasuring.html

I'm a student at MU and I remembered hearing about someone measuring the speed of gravity... and there it is.

But, as far as "instantaneous" transmission goes, there's so much we have to learn about our universe...so never rule it out, never rule anything out.

I think, so far, we humans have realized about 1% of what goes on in the universe. God knows we haven't figured everything out on our own planet.

later...

[buckminster]

Originally posted by chunda21

My question is this - since we are biologically designed to perceive electromagnetic wavicles (light), but not electrogravitational or magnetogravitational waves (unified field theory moots the existence of such), how is it possible to measure it? If you can't measure something, it is not science, it is opinion.
Electromagnetic waves travel at c (sorry for stating the obvious). But, as we are not built to perceive these other, possibly non-existent "waves", I would have thought it impossible to design anything to calculate their rate of propagation. We would not know how to detect it, as all experimental equipment is anthropomorphic - it parallels our senses in some way. Although I would agree that "instantaneous" transmission is highly unlikely if not impossible, there is no way of determining whether or not gravitational waves travel faster than, slower than, or at the same velocity as, electromagnetic waves.

You are absolutely correct on one point. If ya can't measure it, you ain't doing science.

Regarding the method used to determine the speed of gravity waves, you are waaay off. Humans don't need to "perceive" something directly to know that something really exists. We can construct experiments that determine the physical properties of things that are too small to be seen or to determine if something exists or not.

Here is a appropriate example - a long time ago, it was conjectured that light was propagated via a mystical substance called "Ether". Sorta like sound in water. A couple of science dudes tried to prove this "Ether" stuff existed thru an experiment that compared the speed of light in two directions. Guess what they discovered? There ain't no difference. We know now that the speed of light is a constant no matter how its measured. That experiment was a turning point for modern science. It not only established that EMR was not riding on that bogus "Ether" stuff, it led to the revolution that Einstein started with his paper on General Relativity.

This is just one example of indirect methods being employed for measurement of physical phenomena. The current research on quantum gravity is using a technique very similar to the one mentioned above to measure the propagation speed of gravity waves.

You may have been incorrect about measuring stuff but its great that you are trying to understand how scientists think about the universe. Just be careful. The other theologians will be very suspicous of someone who tries to think for himself. That's dangerous, ya know.

best regards,

buck

[eburacum45]

And another link here (http://news-info.wustl.edu/News/casw/suen.html) to Gravity wave detection...
somebody expects to find them, anyway.

_________________________
Sci-fi worldbuilding at
http://www.orionsarm.com/main.html

[Irishman]

buckminster, you are making a common mistake of looking at a model or an analogy, extending the analogy beyond the intended range, then finding flaws in the analogy in the region it isn't claimed to represent the actual case. The rubber sheet model is only intended to give some representation of the warped space concept of gravity. It does not have the proper scale to reflect gravity waves or gravity transmission lag.

chunda21 said:
My question is this - since we are biologically designed to perceive electromagnetic wavicles (light), but not electrogravitational or magnetogravitational waves (unified field theory moots the existence of such), how is it possible to measure it?

We are biologically only able to perceive a small range of electromagnetic waves (light and infrared as heat). However, we are able to build devices to detect the full spectrum, and convert parts to things we can perceive - for instance radio waves into sound and television images. As long as we can find a reason to believe they are possible, and can find a way to convert them to something we can perceive, we can measure it.

[CurtC]

I've always thought that the rubber sheet analogy actually works pretty well to envision gravity waves. Imagine a really big rubber sheet, like hundreds of meters across. Now put on two bowling balls and start revolving them around each other. It seems clear to me that a person far away would be able to detect waves on the sheet due to the motion.

[buckminster]

Originally posted by CurtC
I've always thought that the rubber sheet analogy actually works pretty well to envision gravity waves. Imagine a really big rubber sheet, like hundreds of meters across. Now put on two bowling balls and start revolving them around each other. It seems clear to me that a person far away would be able to detect waves on the sheet due to the motion.

Oh, wow, Curtc, I think you're very right about that one.

I fact I think I finally grok how gravity waves don't replace my cherished notion of the curvature of space-time by mass. That they are just the little perturbations induced in space-time by stuff in motion.

Did I say that right? Hope so...

I think that one of my major mental shortcoming is that I can't visualize shit in my head. I can't spell a word without writing it down. I could never win a spelling bee. I'm a totally brain-dead auto mechanic. I'm so uncreative. and totally inartistic

IFor example, I did a room addition and I needed a 3D CADD package just so I could just visualizer how it would look.

t's like I have this big empty part in my brain that dealt with spatial relationships. Instead, I have this dull plodding linear brain that seems only good for doing computer stuff. Having an almost eidetic memory comes in very pretty handy.

I am just so glad I discovered computers and made a decent living while it lasted.



best regards,

mud
__________________________________________________________
"Our beer is bitter, still, and served with the chill off. There exists a stuff called LAGER so tasteless that it can be cooled without damage and so unsubstantial that a few bubbles make no difference." - English Gentleman

[zimaane]

I am surprised that the article did not mention recent work on determining the speed of gravity by observing Jupiter as it passed in front of a quasar. I would be happy if one of you, maybe even Chronos himself, could give us the straight dope on this experiment. Does it settle the question, and why or why not?

http://www.nature.com/nsu/030106/030106-8.html

[Grey]

http://www.ligo.caltech.edu/ gives instight into the currently operational gravity wave detector in the US. Really exciting and I'd love to hear their first black hole.

[Chronos]

I should have come in sooner, I guess... But the school's connection to the rest of the world was down this morning, so I have an excuse.

So far, gravitational waves are treated as a purely classical phenomenon ("classical", in this context, means "non-quantum"). When we eventually do discover how to quantize gravity (don't hold your breath), "gravitons" will probably be a part of that theory, but as yet, we can only make really vague guesses about what they'll be like, and we'll probably never be able to detect individual gravitons (as opposed to a stream of kazillions of them together, which is what a gravitational wave is), so let's leave quantum mechanics out of this.

Although the rubber sheet model is definitely flawed (as, ultimately, is any analogy), it actually does a fairly good job of describing gravitational waves. Wiggle the masses on the sheet, and the rest of the sheet wiggles, too. As a practical example: Back when I was living with Mom and four cats, I could tell which one of them (the cats, not Mom) jumped onto the bed, from the feel of the waves they spread on the mattress. So different sources can be distinguished by the waves they give off.

There are some things in quantum mechanics which do sort of act spookily at a distance, but it's debateable whether they're really "information". Two electrons, say, can be made to agree in some sense about what their spins are, but you can't say which one of them is "telling" the other, and neither can you use this phenomenon (called "quantum entanglement") to transmit any information which originated outside of the two particles.

Oh, and Dex? Change "finishing" to "starting", and "PhD in astronomy" to "PhD in physics". My bachelor's degree was in astronomy, but I'm in grad school for physics.

[AHunter3]

Well, I've asked this before but not recently, and the answers (or attempts to provide them) might be relevant here, so I'm asking again.

Let's posit a hypothetical universe containing nothing but a hollow sphere, air (earth mixture, 72° Fahrenheit :)) to fill it, and you to either float in it or walk around its inside. Can this sphere be said to be spinning or not spinning? Because if it were to be spinning, inertia would let you walk around on its inside. But relativity and the lack of absolute definitions of speed or direction apply to radial as well as to linear velocity, don't they? So how can the sphere be said to be spinning except in relation to a surrounding universe?

Assuming I'm not on the wrong track so far, let's posit the same sphere except located in deep space somewhere in the middle of our own universe, not particularly close to any galactic clusters and definitely nowhere near any stars or planets or other local gravity sources. Set the sphere, with you in it, spinning in relationship to surrounding universe. Common sense would lead me to imagine that inertia would immediately enable you to walk around the perimeter, but if the underlying definition of "spinning" depends on gravitational interaction with the universe surrounding, is there a lag time? Of course, there is one thing interacting with the sphere right now which conveys the information that the sphere is spinning: the light from the far-away stars in the surrounding universe, emitted billions of years ago, is now striking your sphere from different directions now that you spin. So perhaps gravity-waves do likewise?

So what if, instead of spinning the sphere, we hold the sphere stationary and spin the matter of the universe around it at a corresponding radial velocity instead? (Let us deliberately not spin old photons, unless spinning the stars and galaxies and whatnot drags them along by interaction. The light that left the universe's light sources eons before we gave it a spin presumably continues to stream unperturbed as if the universe had not been given a spin; new light comes from the newly-spinning universe; presumably from the sphere's location, you do not immediately see any sign that the universe is now spinning, because any light reaching the sphere is far far away from that which we just spun -- any light or gravity sources other than the sphere itself -- yes??).

So do you float motionlessly, looking out the window at an unchanging, unrevolving starscape, until the light from distant stars at the time of our sudden spin reaches you billions of years later? Does the gravity / inertia effect reach you at the same time as the visual evidence of the spin?

[Quercus]

AHunter, the problem is that the answer to your first question: "can the sphere be said to be spinning or not?" is "Yes"

Straight-line, constant velocity motion is indeed all relative. No way to say one thing is moving and the other isn't.
But, acceleration is NOT relative. It IS possible to say one object is accelerating and the other isn't. And it is possible, even within a closed system, to tell whether the system is accelerating
And rotating around a point involves constant acceleration (because the direction of motion is constantly changing, even though the speed isn't).

So the sphere is definitely spinning, even without having to relate to an outside universe. And there is a detectable difference between spinning the sphere and rotating the rest of the universe
[obviously, as spinning the sphere is clearly possible, whereas spinning the rest of the universe involves moving things far away at beyond-light speeds and eventually infinite speeds, which is not physically possible]

So your final question doesn't really have an answer, as its a physically impossible situation.

[Tuco]

The link to LISA isn't working - 'You are not authorized to view this page, or it is not temporarily available.'

[White Lightning]

I was just coming in to say that. I was disappointed as it sounded fantastically interesting.

[Punoqllads]

(Relativistic physicist: an ordinary physicist accelerated to nearly the speed of light)

Seriously, though, the thing that I don't get about the speed of gravity is the lack of orbital decay in a two-body system.

Say, for instance, that you have two bodies of equal mass that are orbiting around the point at their midpoint. Now, for a body rotating around a central point, the centripetal acceleration ac = v2/r. The bodies' pull on one another ag = G.m/4r2. For a stable orbit, ac = ag (actually, Fc = Fc, but the masses are identical), so we can solve for v(r) = 1/2.sqrt(G.m/r).

But can we? The gravitational pull from a given object, as your report stated, travels at the speed of light. The first mass' gravitational acceleration vector is not directly opposite of the centripetal, it's from a bit of time before that, however long it took to the gravitational wave to get there. And that means that, in such a case, that the orbit will decay.

Now, if the masses are less evenly matched, then the effects will be smaller, but will still exist. So, for example, the sun's mass is a bit over 1000 times the mass of Jupiter, so the point they orbit about is significantly closer to the sun. But it's still not at the sun's center of mass, so there is some peturbation that, over millions and millions of years, would have caused Jupiter to fall in the sun.

Since this has not happened, my reasoning above is somehow flawed, so I was hoping someone with a better understanding of relativity could explain what I'm missing. The fact that the two bodies are exactly opposite to each other is, of course, true only for the observer who is stationary relative to their center of mass. Am I incorrect about the orbital decay; will it not happen? Is there something about the Lorentz contraction of time/distance that makes the gravitational pull from the opposite mass get there at just the right time? Or is it something else entirely?

[SlowMindThinking]

Seriously, though, the thing that I don't get about the speed of gravity is the lack of orbital decay in a two-body system

Punoqllads, former relativistic physicist here. I've slowed down so much, I'm just a programmer too. And, I frequently make mistakes because I'm trying to write between the interruptions my boss calls work.

General Relativity does in fact predict that orbits decay. One of GR's predicitions is that two orbiting bodies radiate gravitational waves, which travel at the speed of light. Those waves contain energy. That energy comes from somewhere. Chronos mentioned that pulsars provide indirect proof of gravitational waves. That proof is related to the orbital decay. Pulsars are incredibly accurate clocks. So accurate, in fact, that changes in the rate at which they "beat" can indicate orbital decay. All of which answers chunda21's question, I believe.

My question is this - since we are biologically designed to perceive electromagnetic wavicles (light), but not electrogravitational or magnetogravitational waves (unified field theory moots the existence of such), how is it possible to measure it?

And Bucky, I'd be careful being wise with chunda21. I suspect that theologian knows more physics, than you do theology. One thing I learned in graduate school, was that all of those seemingly absurd theological and philosophical types were generally pretty damn sharp.

[foolsguinea]

For some time, my operating hypothesis regarding gravity has been that it is at base a local effect, pulling "space" (whatever that is) into "gravity wells" (whatever they are). The idea that gravity radiates out in particles to pull things in is dubious at best. I think your "waves" of gravity are going to turn out to be the suction in space itself, thus they will travel at whatever speed gravitational acceleration is at that point.

;)The fact that trained physicists don't see this is, of course, Einstein's fault. I don't know how, but always always blame Einstein.:p

[ZenMonkey]

i find it amusing that everyone failed to mention that gravity itself has no speed only acceleration.
anyone remember basic physics.
i believe the number was around 9.8m/s squared.

[SlowMindThinking]

In General Relativity, gravitational waves are distortions in spacetime. They are called waves because the mathematical description of them looks like waves. They move things in and out just like waves. Complicated, tensor waves, but waves. Is that what you mean by "the suction of space itself", foolsguinea? It is the quantum mechanics who picture gravity waves as composed of particles.

I believe (it has been 14 years!!) that it is possible to show that any metric theory of gravity predicts waves. (The term "metric theory" is technical, but it basically means that the distance from a to b is the same as the distance from b to a.)

ZenMonkey, you are remembering the acceleration of an object on Earth due to gravity, not "the acceleration of gravity". Neither gravity nor light ever accelerate. If they exist, they are moving at the speed of light.

[Punoqllads]

ZenMonkey

[buckminster]

Originally posted by SlowMindThinking

And Bucky, I'd be careful being wise with chunda21. I suspect that theologian knows more physics, than you do theology. One thing I learned in graduate school, was that all of those seemingly absurd theological and philosophical types were generally pretty damn sharp.

Oh geez, I hope chunda21 knows that I was just teasing him a little. And I certainly don't think theological and philosophical types are absurd. Those topics might be absurd but then quantum physics is pretty counterintuitive to an outsider, too.

I'm a big buff of the history of science and technology (and Feynman) and have been observing how the scientific community responded as gravity wave research went from being 'little science' to 'big science'. The original GW researchers and their 'low-tech' equipment are treated somewhat like a family reacts to their slightly wierd relative who shows up at gatherings but are reluctant to be associated with them. (I know EXACTLY how that goes..)

At the same time, the scientific community doing GW work has encouraged a diverse set of opinions to be expressed in the scientific journals. They should be given credit for their inclusive attitude. That's why in my initial post, I started out with position that might be considered a 'crank' point of view. That was diliberate, just to generate some discussion.

And slowmindthinking, you are correct about chunda21 knowing more physics than I do theology. That's easy because I know nothing about either topic really. I'm just an old geek who just really wishes he could do computational astronomy or physics.

best regards,

buck
_________________________________________________
"I was born not knowing and have only had a little time to change that here and there."
- Richard Feynman

[RitterSport]

Wait a minute, it takes energy to generate light or other waves or particles, right? Nothing can generate waves or particles forever, right? If gravity is a wave or particle, does that mean the sun will some day run out of gravity? Does its mass decrease over time due to the emission of gravity waves or particles? I'm sure its mass decreases because its emitting lots and lots of energy and particles, but does it also decrease because it's "emitting" gravity?

Or, am I wrong about needing energy to generate waves/particles? Are there other examples of things the can generate that stuff indefinitely?

[CurtC]

Yes, an electron has a charge, but it never runs out. The charge is sent out into space by the photon. This is a pretty close analogy to gravity. No energy is lost.

[Grey]

Tuco/White Lightning give the LIGO site another shot looks like its back.

[RitterSport]

But, doesn't an electron only send a photon out if it changes energy states? So, if it is excited to a higher energy state (from some outside energy source), then falls back down, it emits a photon. Or, if it is shoved up and down inside a wire, it emits photons. It doesn't just sit there and emit photons.

Is that also true for gravity waves? In other words, do you have to wiggle a mass to have it emit gravity waves or particles? Does a lump of iron just sitting there not emit those things? Maybe that's (among) my point(s) of confusion.

[eburacum45]

Originally posted by Punoqllads
The gravitational pull from a given object, as your report stated, travels at the speed of light. The first mass' gravitational acceleration vector is not directly opposite of the centripetal, it's from a bit of time before that, however long it took to the gravitational wave to get there. And that means that, in such a case, that the orbit will decay.


I have heard of this effect before, and some people call it the 'aberration of Gravity' as an analogy to the aberration of starlight.

Not sure SlowMindThinking fully answered the question- as I understand it, this effect would cause orbits to decay in a few millions of years, rather than billions- but unfortunately I don't know the real solution to the problem, although I did have a link to it which no longer works.
From memory it had to do with inertial frames of reference and a compensating force in the other direction,
which allows gravity to be propagated at lightspeed.

as we know, anything which could be used to transmit information
can not be allowed to propagate faster than light, or very bad things will happen.
___________________
Sci fi worldbuilding at
http://www.orionsarm.com/main.html

[CurtC]

RitterSport wrote:
Is that also true for gravity waves? In other words, do you have to wiggle a mass to have it emit gravity waves or particles? Does a lump of iron just sitting there not emit those things? Maybe that's (among) my point(s) of confusion.It is a confusing topic, when you're talking quantum effects. Consider the case of an electron in space by itself, and a proton in space by itself. They're not emitting photons, right? But now start to bring them closer together. There will be some attractive force on the two, and the photon is the medium of this force. As they started to get closer, how did the electron know that the photon was there, so it could exchange a photon with it? Is it sending out photons all the time in every direction, seeing what photons it can interact with? No, that would take energy.

The answer I've heard is that it's continuously emitting "virtual photons," pretend ones that only become real when something is there to interact with.

I don't understand this, but once you accept it, the idea of how quantum gravity might work is a straightforward extension.

[SlowMindThinking]

Rittersport and CurtC, the solution to your conundrum is that static fields doing nothing require no energy to maintain. Consider an infinite universe containing but an electron. The static electric field of that electron is detectable everywhere in that universe, assuming that the electron has always existed. Yes, the electric field is carried by photons, but the electric field is static, which means it doesn't change with time. Since it doesn't change with time, it's frequency is zero (and wavelength infinite). Since the frequency is zero, the wave requires no energy.

Think of a phone cord. It is easy to create a standing wave in the cord; just wiggle it up and down. The less bumps in your standing wave, the longer the wavelength. It takes less energy to create a one bump standing wave than a two bump standing wave. Imagine stretching the phone cord out to infinity, so that you can make really long waves. It still takes less energy to create longer waves than short ones, and obviously it takes no energy to not create a wave - the infinite wavelength limit.

So, an isolated electron can sit around forever, and not lose it's charge. Now imagine the same universe with both an electron and a positron. When one of the electron's virtual photons, which carries that static field, comes in contact with the positron, it is absorbed. Suddenly, that photon has ceased to exist - therefore the field is not static. The energy must come from somewhere, and it comes from the potential energy resulting from the fact that the electron and the positron are in two different places. Reducing the distance between them reduces the potential energy, and supplies the energy for the not static field. This picture presumably applies to gravity also, but without a quantum theory that includes gravity, one should withhold judgement.

However, the gravitational waves that LIGO is designed to detect are different. If you wiggle an electron, it radiates electromagnetic waves. If a negative charge orbits a positive charge, they radiate electromagnetic waves. If you wiggle a mass, it radiates gravitational waves. If one mass orbits another mass, they radiate waves. These waves are both described nicely with classical physics and no quantum is required. (Although the second electromagnetic example what led to Bohr's first quantum model. Classically, an atom out to radiate all the orbital energy away in some mindboggling short time. It takes quantum mechanics to explain an atom.)

[pravnik]

Doesn't the EPR paradox draw doubt on the impossibility of instantanaeity? Did someone in the thread already say this? What exactly is that attractive shiny object on the ground there?

[SlowMindThinking]

A more likely possibility for FTL communication comes from GR itself. GR permits solutions in which traveling back in time is possible. Travelling back in time implies FTL, as you could move, go back in time, move, go back in time, etc, and end up someplace more quickly than light could get there. A negative energy density is required, which is only possible with quantum mechanics, and sending matter through destroys the negative energy "gate". But I haven't seen anything that precludes sending a photon back in time, or at least a virtual photon. And if you can do that, it might mean a LeGuinn like "ansible" is possible.

[fruitcake]

New Scientist magazine had an article about measuring the speed of gravity in January. Here is a link to the article
http://www.newscientist.com/news/news.jsp?id=ns99993232

And the results are that gravity travels at the speed of light.

[Chronos]

As for orbital decays: The effect to which Punoqllads is referring is not the same effect as orbital decay in GR. This happens a lot, that Newtonian physics will predict a result which, by coincidence, seems to resemble a result from GR (as another example, you can "calculate" the Schwartzschild radius of an object by setting the escape speed at the surface to c. You get the right answer, but for the wrong reasons). If you assume Newtonian gravity, then, in order to explain the continued existance of the Solar System you need to suppose that gravity propagates instantly, or at least, insanely faster than light. GR fixes this problem (please don't ask me how), but also introduces an orbital decay effect of its own, which is much smaller and perfectly consistent with the Solar System.

And SlowMindThinking, I wouldn't say that "a negative energy density is only possible with quantum mechanics". It does not seem to be possible with classical physics, to be sure, but we really don't know about the quantum level. Oh, you can get a sort of very localized negative energy density, but in turn, it's not known if that will work for wormholes or any of the other GR "shortcuts". So far as we know, you can produce a wormhole, but if you try to send anything with energy through it, even just a photon, it'll collapse. And while a virtual photon is allowed to have zero energy, you can't transmit any information with a virtual photon.

[SlowMindThinking]

So far as we know, you can produce a wormhole, but if you try to send anything with energy through it, even just a photon, it'll collapse.

Yeah, but the advantage of this method over EPR type effects is that most important phrase "So far as we know...". Perhaps it will be possible to communicate merely by opening and closing wormholes. Actually, I'm sure the uncertainty relations preclude detecting wormholes opening and closing, but given enough clever people and enough time. We can still dream...

[dkoik]

Regarding instantaneous action and its usefulness to transmit information, Slate currently has posted a discussion regarding the principle as applied to quantum computing and quantum cryptography.

http://slate.msn.com/id/2082874/entry/2082873/

[UselessGit]

Doesn´t monarchy travel faster than the speed of light, though? My experiments with what I call "monarchons" have gone off to a slow start, so let me know if you hear about a king with slack security...

[danceswithroaches]

In the 8 minutes it takes for light to travel from sun to earth, the earth moves about 19 arc-seconds through its orbit. Because of this small angle the pressure of the light exerts a small force against the direction of orbital motion. If gravity had a similar lag it would exert an attractive force in the same direction. This force is large enough that the orbital radius would double in about 1200 years. No such effect is observed, so there is no such force. Specially trained smart people have calculated that the speed of gravity has to be at least 10^8 times the speed of light.

Here is the complete explanation:
http://www.metaresearch.org/cosmology/speed_of_gravity.asp

[Chronos]

Yeah, but the advantage of this method over EPR type effects is that most important phrase "So far as we know...".Yes, and I put that phrase in there for a reason. I am not prepared to state that FTL communication is impossible. I am prepared to say that if FTL communication is possible, then it would lead to some other really weird effects, but I'm likewise not prepared to state that those weird effects are impossible, either.

danceswithroaches, the effect you mention is the Newtonian orbit-decay problem mentioned by Punoqllads. The problem does not come up in General Relativity: In GR, the speed required of gravity (both by theory and experiment) is exactly c. I don't have time to fully dissect the link you posted, but I'll start by pointing out that it's not published in any peer-reviewed journal, which means that it's no more reliable a source than any other web page. With a little searching, you can probably find web pages which "explain" gravity as being the result of the great turtle the Earth is riding on inhaling and sucking us all down. But that doesn't make it true.

[buckminster]

Originally posted by Chronos
Yes, and I put that phrase in there for a reason. I am not prepared to state that FTL communication is impossible. I am prepared to say that if FTL communication is possible, then it would lead to some other really weird effects, but I'm likewise not prepared to state that those weird effects are impossible, either.


Hey Chronos,

Are you are just trying to hedge your bet because if FTL anything were possible, we would have to give up some long cherished principles. Ya know, minor stuff, like causality...

Come on, even a post modern deconstructionist astrologer would have to think twice before they are will to kiss that puppy off.

But I agree with your position.

One of the things that separates science from other human activities is that we gotta accept that there is some finite probability that FTL could exist.

I've been reading postings about gravity, cosmology and related topics on Belief.net and that willingness to say 'maybe' is the most fundamental difference.

best regards,

buck
_______________________________________________
"It is far better to grasp the Universe as it really is than
to persist in delusion, however satisfying and reassuring." - Carl Sagan

[Irishman]

Note that the link provided by danceswithroaches is to an essay by Tom Van Flandern. It is most definitely not mainstream science. I don't have the knowledge and training to dissect the article, but FYI, anyone taking this as a statement of the position of cosmologists in general would be mistaken. Just so you know what you're getting into.

[Exapno Mapcase]

I found the Flandern essay interesting myself, but I'm not at all competent to critique it. Other people are and have found it, or at least the peer-reviewed paper on the subject he had published into a physics journal, totally wanting.

Re: Flandern article in Physics Letters A (http://www.lns.cornell.edu/spr/1999-09/msg0017879.html) by Chris Hillman:

I think it is clear that each of these posts criticize different claims made by Flandern in the PLA paper, and each of them dismisses these claims as erroneous or even vacuous. Larry Mead even states the obvious: the referees simply goofed, and let an erroneous paper slip into print. It happens.

[Chronos]

Come on, even a post modern deconstructionist astrologer would have to think twice before they are will to kiss that puppy off.As it happens, I am not a post modern deconstructionist astrologer ;). But seriously, discarding causality is one of those Really Weird side effects I mentioned. I'm a little more willing than most to reject causality (but still not without some good evidence!), although I admit that that may be partially wishful thinking.

[Grey]

Originally posted by danceswithroaches
In the 8 minutes it takes for light to travel from sun to earth, the earth moves about 19 arc-seconds through its orbit. Because of this small angle the pressure of the light exerts a small force against the direction of orbital motion. If gravity had a similar lag it would exert an attractive force in the same direction. This force is large enough that the orbital radius would double in about 1200 years. No such effect is observed, so there is no such force. Specially trained smart people have calculated that the speed of gravity has to be at least 10^8 times the speed of light.
Now wait a minute. Are you telling me that the effect of the sun’s mass on the fabric of space resets to zero unless there is an actual mass there to interact with? The warping of space by the sun is present regardless of the presence of the earth. The warping changes slightly as the two gravity wells move through each other but I fail to see why you would need to worry about this. What did I miss?

[buckminster]

Originally posted by Exapno Mapcase
I think it is clear that each of these posts criticize different claims made by Flandern in the PLA paper, and each of them dismisses these claims as erroneous or even vacuous. Larry Mead even states the obvious: the referees simply goofed, and let an erroneous paper slip into print. It happens.


One of the unusual aspects to this line of research has been the inclusive manner that the 'fringe' element has been treated by the mainstream researchers. This is a good thing. Yeah, Flandern has some strange opinions. And the first generation of scientist who tackled gravity wave detection are no longer in the loop any more either. Overall, society and science is better served by giving these fellows an opportunity to be heard and participate.

I seriously doubt that Flandern's theories have any validity. but if scientist don't address these unusual viewpoints, then these fringe elements seem to get a big boost from the conspiracy, UFO, ancient astronaut, ESP, pyramids built by aliens folks. I wonder if astronomers had addressed the theories of Velikovsky (http://skepdic.com/velikov.html) 50 years ago, would anyone would bother to attend an event such as this symposium (http://www.knowledge.co.uk/sis/50years.htm). Maybe I'm being optimistic (a serious fault of mine) but I like to think that most people are reasonable and they would start to catch on to these guys after awhile.

best regards,

buck
_________________________________________________
"If we don't believe in freedom of expression for people
we despise, we don't believe in it at all."- Noam Chomsky

[Chronos]

On the one hand, there is value in letting the fringe elements have their say, but on the other hand, you can't address all of the fringes. The problem is that there are far too many of them. If professional physicists and astronomers (and I assume this is true of other scientists as well) addressed every "theory" which crossed their desks, then they wouldn't have time to do anything else at all. One has to draw a line somewhere, but unfortunately nobody seems to agree on where.

The presentation of an idea also makes a big difference. If a person writes a paper saying that "Assuming a Newtonian model of gravity, analysis of motions in the Solar System indicates a speed of gravity of at least 109c", then at worst the paper will be regarded as interesting but irrelevant. If nothing else, such a paper can be used as evidence against a Newtonian model. If, though, the person writes a paper saying "Our analysis of Solar System motions proves that gravity must have a speed of at least 109c", then it's hard to say that the paper is anything other than just plain wrong.