PDA

View Full Version : The Speed of Gravity

mopc
2005-Feb-19, 12:55 AM
Let's say our Sun compresses itself into a black hole density... how long would it take for the earth to feel? What about Uranus???

I mean, my doubt is does gravity have a "speed"? I mean, if a gravitation field changes, how fast does it spread into the universe???

2005-Feb-19, 01:25 AM
It's still the speed of light in a vacuum, c. I remember something about a graviton being transferred between the two masses, and these particles move at the speed of light, but I'm not sure..

W.F. Tomba
2005-Feb-19, 01:53 AM
I think if the Sun compressed itself to black-hole density, we wouldn't feel anything. The Sun's mass would remain the same, as would the distance from here to the center of the Sun.

My very rudimentary understanding of the subject is as follows: according to General Relativity, gravity is the result of curvature in spacetime caused by nearby masses, so it does not actually take any time for the Sun to attract you. It's not a force being transmitted through space from the Sun to you; it's an effect of the spacetime geometry right where you are. But changes in the position and magnitude of masses cause changes in the local spacetime curvature. These changes propagate outward at the speed of light.

So if the Sun suddenly vanished, Earth would stop feeling its "pull" some eight minutes later.

dgavin
2005-Feb-19, 02:14 AM
Well from my understanding of Gravity it's still a large unknown, but there are Two major theroies about it.

In Quantum terms when they decribe Gravity it's via a thoretical carrier called a Graviton which does obey 'c' speed. All attempts to detect said particle have failed thus far. (The Gravity Wave Detector is the latest in trying to prove this form of gravity)

However if Gravity is truely what it seems to be (warping of space via Mass/Energy, or both) then the effect of these warps changing can propagate at faster then 'c'. Abercombies warp drive is based on this priciple, and there is some evidence that a rare galaxy or two may be moving at superluminal speeds (that is thier Space Time is moving faster then C, which is the major reason some have started calling Galaxies, "Island Universes")

But to answer your Question if the sun collapsed into a Black Hole, Earth would still fell the same amount of gravity from it. Gravity in a black hole doesn't somehow multiply. What happens is the Matter that warps space becomes so dense, that light cannot escape the effects of the warp.

All gravitational bodies have this event horizon 'mathmatically', but the event horizon is 'inside' the gravitational body and is typically smaller in size then an electron, it's when it's outside of the gravitational body that you have what is known as a black hole.

I subscribe to the second understanding that Gravity is simply a warping of space/time and not some particle. If this is the one that is proven it will mean two things. No Grand Unification Theroy (as there is no Quantum of Gravity) and two, that gravitational effects propogate faster then 'c'. This is why the second one is not as popular, because it technically violates GR understanding and we humans just love to quantify things.

TravisM
2005-Feb-19, 03:26 AM
[snip]However if Gravity is truely what it seems to be (warping of space via Mass/Energy, or both) then the effect of these warps changing can propagate at faster then 'c'. Abercombies warp drive is based on this priciple, and there is some evidence that a rare galaxy or two may be moving at superluminal speeds (that is thier Space Time is moving faster then C, which is the major reason some have started calling Galaxies, "Island Universes")

Woah. #-o This just hurts to read.
I will only take a crack at one of these.

Gravity, as we know it in either theory (QM or GR) acts at the speed of light.

Island universes was a term that had been used prior to 1927...
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...

Sam5
2005-Feb-19, 03:35 AM
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...

Psst, it's "super-luminal".

Fortis
2005-Feb-19, 03:41 AM
Abercombies warp drive is based on this priciple
I think that you mean Alcubierre. I suspect that Abercombie (and Fitchs) warp drive would also have a corresponding weft drive as well. ;) :)

2005-Feb-19, 03:48 AM
Yeah, if it collapsed into a black hole, gravity would only increase when you enter the previous radius of the sun.

TravisM
2005-Feb-19, 04:18 AM
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...

Psst, it's "super-luminal".

Psst, I know... :roll:

John Kierein
2005-Feb-19, 01:47 PM
I think gravity is a push from external long wavelength electromagnetic pushing masses together. Thus the graviton is a quantization of the shadow cast by the mass. So gravity travels at the speed of dark.

See the chapter I wrote in the book 'Pushing Gravity' edited by Matt Edwards available on Amazon and elsewhere.

Normandy6644
2005-Feb-19, 04:47 PM
And no galaxy is 'super-luminous,' it only appears as such due to the expansion of space...

Psst, it's "super-luminal".

Psst, I know... :roll:

I thought it was "super-liminal." Uh oh. :D

mopc
2005-Feb-19, 05:30 PM
Ok, so basically i can say that "gravity travels at the speed of light".

Like in the example given above, "if the Sun suddenly vanished, the Earth would stop feeling its pull some eight minutes later".

But nothing actually "vanishes" or "appears" as if by teleportation. How can anyone actually know??? Or people just assume gravity is as fast as light because light is the maximum anyway?

Well, maybe gravity can be measured by just moving an object, like as the moon revolves around the earth we could measure the "lag" between the motion and its gravitational effect on earth (tides, etc.)????? Is that how they measure the "speed" of the space-time geometry deformation caused by mass that we call gravity?????

Matt McIrvin
2005-Feb-19, 07:32 PM
The question is often posed "when would we feel it if the Sun suddenly vanished?" But, as you say, this simply doesn't happen in general relativity, so general relativity isn't going to provide a coherent answer to the question.

What kinds of gravitational disturbances can happen? Changes in the shape of an object, for one: the simplest kind would be a change in the shape of the sun (due to some enormous internal eruption, say) that caused it to become more ellipsoidal, or to oscillate between a flattened and a football-shaped ellipsoid. General relativity says that the effect of that would propagate to Earth at the speed of light.

So it's fair to say that gravitational effects propagate at lightspeed in GR. Now, that does not mean that the Earth feels itself pulled toward the "retarded position" of the Sun where it was eight minutes ago. (Indeed, in a relativistic world it's hard to say what "the place where the sun was eight minutes ago" would even mean: according to what reference frame?) In a sense, the Sun's gravitational field is freely falling right along with it, and as a consequence the pull on the Earth is much closer to pointing to where the Sun is now. But if the Sun, say, suddenly blew apart into two pieces that went shooting off in opposite directions, the Earth would keep trucking along in its former path for eight minutes.

Bathcat
2005-Feb-20, 02:35 AM
Yes, even though the Sun is speeding along its galactic orbit and gravitational changes due to a sudden change in the Sun's position would take 8 minutes to affect Earth...still the vector of attraction at Earth points to the position of the Sun right now and not its position 8 minutes ago.

As I recall, mathematical physics requires that this be so if the conservation laws are correct.

Commonly spacetime warpage is pictured as a dimple in a rubber sheet, in the case of the Sun and Earth, a Sun-dimple with the Earth rolling around the inside of the dimple (and the Moon rolling around the Earth's dimple).

One way to visualize the vector of attraction is to think that, if the Sun is considered in motion, then the gravity-dimple it makes is slightly lopsided -- the farther you get from the center of the dimple the more the "downhill" vector points slightly in advance of the Sun's position at Time T.

And it turns out that the adjustment in the "downhill" direction is such that at 8 light-minutes from the Sun the "downhill vector" points at the spot where the Sun will be in Time T + 8 minutes.

So in this view, yes, the Earth responds to the Sun's gravity because the Sun's gravity-dimple is already in place where the Earth is.

But the gravity-dimple only adjusts to smooth, regular motion -- like ORBITAL motions! So if you suddenly jerked the Sun 20,000 kilometers to the Galactic North, the effect -- travelling as gravity waves, ripples in spacetime -- would not reach Earth for 8 minutes.

And now, consider what real-world mechanism would give the Sun a non-orbital, non-smooth jerk...and that's why we don't see gravitational anomalies except in very special conditions like closely-orbiting neutron stars!

[addendum!] This is a visualization only, not a rigorous explanation. There are some good papers on the Web if one wants to really know what's going on. Have pity -- I only mean to give a layman's gloss on the subject.

mopc
2005-Feb-20, 03:02 AM
This stuff is boggling my brain! Ok, the vector of attraction points at the sun right now, so technically the "information on gravity" is instant. But what if the Sun takes an unusuall unpredicted path? Will the earth feel the crazy movement also instantly? Or maybe someone already answered this question?

Maybe we can transmit information instantly through gravity field in that case. Boy I'm in for a nobel. :o

Matt McIrvin
2005-Feb-20, 03:49 AM
This stuff is boggling my brain! Ok, the vector of attraction points at the sun right now, so technically the "information on gravity" is instant. But what if the Sun takes an unusuall unpredicted path? Will the earth feel the crazy movement also instantly? Or maybe someone already answered this question?

The transmission of the information isn't instant-- it's just that there is a compensatory effect that makes up for the delay, as long as the sun follows along a freely-falling path and doesn't do anything else. As I said above, if the sun were to suddenly change shape, or split in two, or start jetting out matter and rocket off in some crazy direction, we wouldn't feel that until eight minutes later.

A similar thing actually happens even in electromagnetism-- though there, it is a little different and only applies to straight-line motion at constant speed. If a charged particle is moving in a straight line at constant speed (and has been moving like that for long enough), the electric force produced by its electric field actually points straight toward its current position, even if it's far away. But if the particle makes a sudden left turn, that information only propagates out at finite speed. (And if you shake it back and forth, the fields make a radio wave...)

Bathcat
2005-Feb-20, 04:48 AM
Yes, I agree entirely, Matt! And as I recall, the equations of GR are of a higher order (mathematically) than the equations describing electromagnetism.

Because of that difference in order, the retardation, the adjustment for motion, that Matt describe for an electron in unaccelerated motion applies in gravitation to systems under smooth accelerated motion as well as unaccelerated motion. (Orbital motion is accelerated motion.)

(No, electrons do not 'orbit' an atomic nucleus the way the Moon orbits the Earth, and they are not therefore forced to radiate away all their energy in radio waves, as Matt most succinctly describes a 'shaken' electron doing. Electron orbitals are resonant quantum patterns in which the electron waveform resides. Sort of, in a manner of speaking.)

And no: you cannot transmit information superluminally using gravity. The gravitational field is either already at Earth -- the usual case, which causes the vector of attraction toward the Sun to adjust itself toward the Sun's advanced position and not the retarded one. Or else the waves caused by gravitational change propagate at the speed of light.

The advanced vector of attraction -- the fact that with respect to the Galaxy the Earth is orbiting around the position of the Sun right now -- is giving us information about the Sun 8 minutes ago: it tells us that the Sun was at position X,Y,Z and in motion at Time T - 8 minutes.

It does NOT tell us that the Sun will definitely, assuredly, and without a doubt actually arrive at the place the Earth is being attracted to. Nosir! If the Sun were to magically disappear, the Earth would continue to orbit the position of the non-existent Sun for 8 minutes. No information would be transmitted superluminally.

Now, the question of reference frames makes things look funny. If you use a reference frame in which the Sun is motionless, and all the planets move around the Sun, then there is no distortion of the gravity-dimple and the planets all move around the unaccelerated, unmoving Sun. If you take into account the Sun orbiting the Galaxy, the gravity-dimple distortion described above comes into play. And if you take into account the Galaxy's motion through the Local Cluster, then the gravity-dimple is distorted to account for that motion! Even with all those apparent motions, the vector of attraction points toward the Sun's position at time T and not at T - 8 minutes.

(Please, please note carefully that in the paragraph above I am referring to T - 8 minutes as experienced by Earth. In my previous post, I referred to T + 8 minutes as experienced by the Sun. Sorry I am so darned wishy-warshy.)

But it's not really odd at all. The laws of physics look the same no matter what the observer's state of motion, and so whether you define your observer as stationary with respect to the Galaxy or stationary with respect to the center of gravity of the Local Cluster, the calculations will always have Earth's vector of attraction pointing to the advanced position of the Sun.

dgavin
2005-Feb-20, 05:04 AM
Hate to point this out, but there is no evidence yet that gravity effects are limited to 'C'.

If one of the reasons fo the gravity wave detectory. If Gravitatioanly waves can be detected then it would be contrained by 'GR'. If they arn't...

Until we know though you can't say that gravity is limited, or not limited to light speed.

It's still an unknown.

Bathcat
2005-Feb-20, 05:13 AM
It's reasonable to say that, I suppose.

GR predicts gravitational waves, and some systems have been discovered which GR predicts would produce gravitational waves. The systems are closely orbiting neutron stars. The waves predicted by GR would carry energy out of the orbital system, causing measurable degeneration of the orbits.

This has been observed, and the orbital degeneration matches the predictions of GR for the amount of energy which should be carried out of the system by gravitational waves.

So there is, I think, indirect evidence for G-waves as predicted by GR. GR has not, in the regime of large bodies and large motions, been falsified.

It's hoped that the gravity observatories will not only detect these waves but give us details about the high-gravitational regime.

But certainly gravitational waves so far remain undetected.

mopc
2005-Feb-20, 08:59 PM
Hate to point this out, but there is no evidence yet that gravity effects are limited to 'C'.

If one of the reasons fo the gravity wave detectory. If Gravitatioanly waves can be detected then it would be contrained by 'GR'. If they arn't...

Until we know though you can't say that gravity is limited, or not limited to light speed.

It's still an unknown.

Okay, then, if this is correct my question remains ananswered? :-k

What the heck is gravity any way? In quantum, it's a frigging particle that no one has ever seen, in GR it's space-time geometry distorted by mass.

But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?

The GR explanation seems more plausible, even though it's not actually an explanation, just the stating of the rules of a phenomenon. No one knows exactly why gravity exists, do they?

dgavin
2005-Feb-21, 05:04 PM
Hate to point this out, but there is no evidence yet that gravity effects are limited to 'C'.

If one of the reasons fo the gravity wave detectory. If Gravitatioanly waves can be detected then it would be contrained by 'GR'. If they arn't...

Until we know though you can't say that gravity is limited, or not limited to light speed.

It's still an unknown.

Okay, then, if this is correct my question remains ananswered? :-k

What the heck is gravity any way? In quantum, it's a frigging particle that no one has ever seen, in GR it's space-time geometry distorted by mass.

But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?

The GR explanation seems more plausible, even though it's not actually an explanation, just the stating of the rules of a phenomenon. No one knows exactly why gravity exists, do they?

No it's less likely it's an Aether then one of the other two. Although if you were to approachit from a purely research stand point, you couldn't rule that out yet either.

As one gent pointed out, Gravity seems to follow GR contraints in some cases, but not in others.

There was a resent show on Nova about a galaxy or two that were super massive and thier velosity was superluminal (they wern't talking appearant motion either) so it seems space time itself doesn't obey GR contraintis. Some have started calling this behavior 'Gravitational Detachment' as space time seems to move differently around these objects.

Grey
2005-Feb-21, 06:04 PM
Okay, then, if this is correct my question remains ananswered? :-k
Well, if you want to be pedantic. But you could also say that our best theory of gravity, general relativity, which has held up to a mountain of tests, predicts that gravitational disturbances propagate at the speed of light. And though we don't have a complete quantum theory of gravity, we know enough about the form such a theory would have to take to know that it, too, would predict that gravity propagates at the same speed as light. It's possible of course that both of these will be completely overturned, but just saying "we really don't know" implies that it's equally likely either way, and that's really not accurate.

But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?
No, the graviton would be a massless particle, just like the photon. It would however (also like the photon) have energy associated with it, and so could produce its own gravity (it's really energy that couples with gravity, not mass), which is one of the things that makes gravity more complex than electromagnetism (photons don't have charge). As for no one measuring or detecting it, it's probably just a matter of time and more precise detectors. Since gravity is so weak, comparatively, gravitons are hard to detect. It was a long time before we were able to make detectors capable of spotting the elusive neutrino, too.

Sam5
2005-Feb-21, 07:06 PM
No, the graviton would be a massless particle, just like the photon. It would however (also like the photon) have energy associated with it, and so could produce its own gravity (it's really energy that couples with gravity, not mass), which is one of the things that makes gravity more complex than electromagnetism (photons don't have charge).

Let me mention something that I’ve been researching during the past year, which is the origin of the words “particle” and “photon” in association with “light”. Actually, I think the English word “particle” is not a good word to use. Several German-language writers in the 19th and early 20th Century used three different German words that basically meant “part”, “smaller part”, and “smallest part”. Often these words were translated into English as “particle.” I think this might possibly have been due to the German use of the German words for “part”, “smaller part,” and “smallest part” in reference atomic “particles” like electrons and protons. But the German words would have meant something slightly different in both cases, i.e. for light and for atomic parts. For example, a “particle” of an atom might be considered to be a “little round thing”, while a “particle” of light might be considered to be a “long little group or bundle of waves”, or perhaps “one single wave cycle that has a specific wavelength”.

Planck, who was one of the first people to write about what we now call a single “photon,” used one of the German words around 1900, in reference to light, but early translations of his paper did not use the word “particle”. It used the English word “part”. The early English versions of the German terms Planck used in his “photoelectric effect” papers were: “discrete quantity” and “the energy element ε”. These terms have since been referred to in many English papers and books as “particle,” “quantum”, and “photon”, leading some people to believe they are “little round things”, rather than a very small wave or wave train of EM waves. The “photon” word didn’t even come along until someone invented it in 1926, so it was not used in Planck’s or Einstein’s early papers on this subject.

I have an English translation of Einstein’s famous 1905 “photoelectric effect” paper, and it does not contain the word “photon”, and I can’t find the term “light particle” in it. The English translation of his first main explanation reads like this: “consists of a finite number of energy quanta that are localized in points in space, move without dividing, and can be absorbed or generated only as a whole.”

I’ve learned enough from my research of English translations of old German papers to know that we really need to see Einstein’s original German text to determine what he actually meant by the term “points in space”. I think he might have meant a “point” where a wave front is located and another point behind that where the “wave end” is located, regarding a single “energy quanta” that was made up of EM waves and that had a wavelength. I’m not sure he meant that a single “energy quanta” could be described as being at a single point in space, and I’m sure he knew in 1905 that Planck was not talking about a point particle, but, essentially, he was talking about the smallest shortest bundle of electro-magnetic waves of light that an atom could emit, since both Plank and Einstein tended to use the Maxwell model of light waves at that time. In other words, a Maxwell-type short “wavelet” or the shortest “wave train” or perhaps just one full cycle of a Maxwell electric and magnetic wave traveling through space would have been the “the energy element ε” that Planck was talking about in 1901 and what Einstein called “a finite number of energy quanta” contained in a “ray” of light in his 1905 paper.

We can study Einstein’s paper and Planck’s paper and think of their view of a “quantum” as the smallest group of waves a single light ray or beam could be broken up into, and a single “photon” being the shortest emission of light waves by a single atom. The word “photon”, in my opinion, which they did not use, misrepresents their own ideas about what a light “quantum” was. This was not their fault, but it represents a modification (and I think a misinterpretation) of their original ideas by later science writers. The way a single “photon” of light can actually be an EM “wave” yet act like a “particle” is in the way the wave hits and resonates an atomic absorber. An EM wave can be a true “wave”, but it can hit an atom with a force of energy, and without being a physical “little round thing” type of “particle”.

For example, you can set a small round magnet on its edge on a table top, with its South pole aimed to the right, and you can pass another small round magnet by it, with its South pole pointing left, and you can knock over the first magnet without either of the magnets touching each other. The energy force that knocks over the magnet is strictly a “force field”, but not a “particle”, unless, perhaps, QM scientists think a magnetic field is made up of very tiny little “particles”, but, personally, I don’t have that particular belief at this time.

Fortis
2005-Feb-21, 07:18 PM
The energy force that knocks over the magnet is strictly a “force field”, but not a “particle”, unless, perhaps, QM scientists think a magnetic field is made up of very tiny little “particles”, but, personally, I don’t have that particular belief at this time.
QED?

dgavin
2005-Feb-21, 08:15 PM
Okay, then, if this is correct my question remains ananswered? :-k
Well, if you want to be pedantic. But you could also say that our best theory of gravity, general relativity, which has held up to a mountain of tests, predicts that gravitational disturbances propagate at the speed of light. And though we don't have a complete quantum theory of gravity, we know enough about the form such a theory would have to take to know that it, too, would predict that gravity propagates at the same speed as light. It's possible of course that both of these will be completely overturned, but just saying "we really don't know" implies that it's equally likely either way, and that's really not accurate.

But if it's a particle ("graviton") than it can have mass, then gravity has it's own gravity field? :-s . Okay then it's a particle with no mass? That no one measures? That no one detects? Than isnt it Sagan's "invisible dragon in the garage"? An "aether"?
No, the graviton would be a massless particle, just like the photon. It would however (also like the photon) have energy associated with it, and so could produce its own gravity (it's really energy that couples with gravity, not mass), which is one of the things that makes gravity more complex than electromagnetism (photons don't have charge). As for no one measuring or detecting it, it's probably just a matter of time and more precise detectors. Since gravity is so weak, comparatively, gravitons are hard to detect. It was a long time before we were able to make detectors capable of spotting the elusive neutrino, too.

There is some more to think about this way, if Gravity is a paritcle that carries the force, does it have an anti particle (which would be probable), and does that explain the pushing of galaxies away from the empty area's of the universe? This GR consept of gravity does fit with what we know at current.

If Gravity is simply a warp of space time by the presence of something (no better way to explain it) and not a force, then it fits the explaintaion of the few gravitationaly detached faster then 'C' galaxies.

Could it be a combination where the Warping of Space by mass/energy has the effect of generating virtual or real particles that are gravitons?

Thats the main issue is that Gravity as we know it seems to exhibt both GR sub 'c' behavior, and warped space behavior.

Which might be a good indication it could have a duality nature like some other things in QM. But lets not go into that as that is ATM thinking.

Althoug looking at it this way, duality gravity would allow both models we know of to fit together.

Kaptain K
2005-Feb-21, 09:07 PM
The graviton, like the photon, is its own antiparticle!

mopc
2005-Feb-21, 09:21 PM
Let me mention something that I’ve been researching during the past year, which is the origin of the words “particle” and “photon” in association with “light”. Actually, I think....

As a translator of German myself, I understand this always difficult task of precise translation, since the semantic field of words in two languages rarely coincide. Imagine how much of science worldwide is misunderstood because of language mismatch!

But returning to the topic, I can thus say that gravity "travels" at the speed of light except under certain special conditions, when it can be faster, right? 8-[

Sam5
2005-Feb-21, 09:35 PM
As a translator of German myself, I understand this always difficult task of precise translation, since the semantic field of words in two languages rarely coincide. Imagine how much of science worldwide is misunderstood because of language mismatch!

But returning to the topic, I can thus say that gravity "travels" at the speed of light except under certain special conditions, when it can be faster, right? 8-[

Frankly, I don’t know, but from my own “classical” perspective, I would think that gravity “should” travel at some speed, and the speed of light is ok with me.

Regarding gravity “waves”, I think what that means is actually more like a “gravity tsunami”. Not a long series of continuous waves like light, but a sudden big “wave” of change, if the body in question suddenly moves or blows up. I think that is the kind of “wave” most people have been searching for. If bodies like rapidly revolving binaries continue to revolve, then they might send out a long series of changing gravity “waves” that are related to their changing positions.

Sam5
2005-Feb-21, 09:41 PM
QED?

I like to think of things like gravity and magnetic fields as being “continuous”, but the QM guys like to talk about everything in terms of quanta and “particles”. So, maybe what I think of as a smooth and continuous gravity and magnetic field is actually made up of little things that are so small, I just don’t feel their roughness or their individual bumps. It’s like, even honey and molasses are supposed to be made up of individual molecules. So maybe if we keep examining the smallest of scales, even gravity and magnetic fields are made up of a lot of little individual things.

Grey
2005-Feb-22, 05:02 AM
I like to think of things like gravity and magnetic fields as being “continuous”, but the QM guys like to talk about everything in terms of quanta and “particles”. So, maybe what I think of as a smooth and continuous gravity and magnetic field is actually made up of little things that are so small, I just don’t feel their roughness or their individual bumps. It’s like, even honey and molasses are supposed to be made up of individual molecules. So maybe if we keep examining the smallest of scales, even gravity and magnetic fields are made up of a lot of little individual things.
For the magnetic field, we know that's true. Well, "know" in the sense that our best available theory (quantum electrodynamics, as Fortis pointed out), which has passed all tests thus far thrown at it and shown amazing success, says that's the case. The electromagnetic force, and thus any effects ascribed to it, such as the forces caused by electric or magnetic fields, is the result of exchange of virtual photons. At the smallest scales of interaction, it is necessary to take into account this "graininess" of the EM interaction to make the right predictions, so it's quite likely that any successor to QED will also incorporate it. We expect the same to be true of gravity, but with the relative weakness of gravity as compared with the other forces, we have yet to show that directly by experiment. I don't think there are any physicists who will be surprised by its eventual discovery in the coming years, though the experimenters on that project will doubtless be on the short list for a trip to Stockholm.

Inferno
2005-Feb-22, 05:05 AM
They haven't actually proved the existence of the graviton yet, have they?

Grey
2005-Feb-22, 05:10 AM
They haven't actually proved the existence of the graviton yet, have they?
No, but that's no surprise. A graviton has such a low energy that it's very hard to detect, and our current detectors are just now reaching the point where they even have a chance of doing so. The trickiest part is trying to spot a graviton over the noise of, say, a truck driving down the highway a mile away. Give 'em time. :D

Sam5
2005-Feb-22, 05:41 AM
The electromagnetic force, and thus any effects ascribed to it, such as the forces caused by electric or magnetic fields, is the result of exchange of virtual photons. At the smallest scales of interaction, it is necessary to take into account this "graininess" of the EM interaction to make the right predictions, so it's quite likely that any successor to QED will also incorporate it.

Can you explain what a “virtual photon” is?

kenneth rodman
2005-Feb-22, 06:29 AM
LETS SIMPLIFY this. If the sun for some unknown reason suddenly changed its course, depending how fast it did so would determine the effects on earth.

imagine a big ball pulling a smaller ball with it, and they are attached buy a rubberband:

I know its a poor drawing but you get the idea. the small ball is circling the big ball the rubberband represents the force of gravity. a sudden movement would cause one part of the band to pull harder than the other part of the band and cause a wobble till after how many sets of revolutions it balanced itself back out. Too hard a yank and you might snap the band. AT least thats how i visualize it if im wrong let me know. Oh if the big ball was suddenly not there then there would be nothing pulling on the rubberband towards the suns direction so fling out into the depths of space our little planet would go.

scourge
2005-Feb-22, 08:55 AM
How does mass curve spacetime? What’s the mechanism? Can the effect be artificially produced somehow?

I think gravity is a push from external long wavelength electromagnetic pushing masses together. Thus the graviton is a quantization of the shadow cast by the mass. So gravity travels at the speed of dark.
How does this model explain frame-dragging and time dilation, John?

LETS SIMPLIFY this. If the sun for some unknown reason suddenly changed its course, depending how fast it did so would determine the effects on earth.

imagine a big ball pulling a smaller ball with it, and they are attached buy a rubberband:
I think I see what you’re getting at, and it won’t work—this is akin to the idea of ‘if you have a rigid rod a light-year long, won’t the force you transmit by pushing on it move faster than the speed of light?’ It turns out, no, it’ won’t—nothing is so rigid that it can transmit a compression wave faster than the speed of light.

I think it’s helpful to consider C as ‘the speed of reality,’ that is, if the Sun suddenly vanished right now, that ‘reality’ is still eight minutes in our future. It’s crude, but it simplifies these kinds of questions, in my mind anyway.

kenneth rodman
2005-Feb-22, 01:45 PM
THE RUBBERBAND represents the force of gravity so t speak. the small ball(earth) is orbiting the big ball(sun) . IF you spin a ball attached to a string, the ball keeps the string tight because its trying to move away from you, the string keeps it from doing so. the string in this case is the force of gravity. If suddenly the string disapeared, The ball flys off. I used a rubberband because it can strech and still pull an object back towards it.
Numerous things could happen in our hypothetical senario: the orbit could become unstable, be permantly changed into a more elipitcal one, or it could wobble for awhile and then return to its current state all depending on how the sun suddenly move and how swiftly, ect. I kind of zeroed in on this part of his question:

If the sun suddenly vanished instead of just altering its trajectory then
IT would be 8 minutes later when the effect of uch an event would be noticed by us.
But what if the Sun takes an unusuall unpredicted path? Will the earth feel the crazy movement also instantly? Or maybe someone already answered this question?

I was focusing on how the planet could be effected not as to when we would experience the event. in all the senarios would still take 8 minutes before the effects were felt by us so to speak, and Going back to the very first question If the mass of the sun stayed the same just suddenly became a black hole our orbit would remain unchanged but 8 minutes later or so we would notice it was a bit dark. :)

scourge
2005-Feb-22, 02:43 PM
I was focusing on how the planet could be effected not as to when we would experience the event. in all the senarios would still take 8 minutes before the effects were felt by us so to speak, and Going back to the very first question If the mass of the sun stayed the same just suddenly became a black hole our orbit would remain unchanged but 8 minutes later or so we would notice it was a bit dark. :)

Oh. Sorry 'bout that. Yes, dark..and soon after, cold. Brutally, deadly cold within a few days, I'd figure.

Then we'd all become Morlocks :evil:

Grey
2005-Feb-22, 05:26 PM
Can you explain what a “virtual photon” is?
Sure. How technical should I get? Let's start with a lay explanation, and if you'd like something more precise, feel free to ask. Just like the uncertainty principle limits how precisely you can know the momentum of a system depending on how precisely you know its position, it also prevents you from knowing the energy of a system precisely, depending on how short a time scale you're concerned with (the shorter the time scale, the larger the uncertainty in the energy). So a photon can pop into existence, borrowing energy from the vacuum, as long as it vanishes again and pays back the energy quickly. Under quantum electrodynamics, all electromagnetic interactions consist of a particle emitting a virtual photon, which then interacts with a second particle, transferring momentum and energy from one to the other.

Sam5
2005-Feb-22, 05:51 PM
Grey,

Hmm... mumble, grumble, think, think.....

Is light emitted when this process takes place? Why don’t we just call it a “photon”?

frogesque
2005-Feb-22, 06:08 PM
Something that's been nagging at me on and off for a while and might be better in ATM but I'll float it here first since it's relevent.

If gravity behaves as a wave should it be possible to difract the wave though a 'slit' and detect interferance zones? (the analogy is difraction of light)

Grey
2005-Feb-22, 06:44 PM
Is light emitted when this process takes place?
No. We can't detect virtual particles normally. Any photon detector relies on the photon imparting energy to something else through some mechanism or another. If a virtual particle did that, it couldn't "pay back" the energy it had borrowed, and we'd actually have to account for energy being created or destroyed.

Why don’t we just call it a “photon”?
Well, it's not really "real". Virtual particles of any type don't necessarily have to follow all the rules that real particles have to follow. For example, they can have negative energy or momentum. So we have these ghostly "particles" that we can't observe appearing out of nowhere and then vanishing, carrying momentum and energy that we see as electromagnetic interactions. The strong and weak forces are similarly carried by gluons or W and Z particles). Moreover, interactions can get really complex, since a virtual photon could appear, decay into a virtual positron and electron pair, which could then annihilate to produce a photon again, which then vanishes.

Like many aspects of quantum mechanics, it sounds so weird that nobody would take it seriously, except that it does so amazingly well at predicting what we see! For example, if virtual particles really exist, it's possible to show that atomic energy levels would be slightly shifted from what they would otherwise be. Similarly, there would be a slight difference in the expected magnetic moment of the electron. When we look, we actually see exactly these differences, to a really remarkable level of precision. I think that the results currently agree to something like one part in 10^8.

Sam5
2005-Feb-22, 07:05 PM
Like many aspects of quantum mechanics, it sounds so weird that nobody would take it seriously, except that it does so amazingly well at predicting what we see! For example, if virtual particles really exist, it's possible to show that atomic energy levels would be slightly shifted from what they would otherwise be. Similarly, there would be a slight difference in the expected magnetic moment of the electron. When we look, we actually see exactly these differences, to a really remarkable level of precision. I think that the results currently agree to something like one part in 10^8.

Hmm....

Ok, I think that’s a pretty good explanation.

So, would you say that theorists use these kind of invented terms and hypothetical “models”, because they seem to work out, although they might not actually be a completely accurate explanation of what is happening, but later, hopefully, physicists will figure out what is actually happening, but right now they’ll use these tentative concepts because they seem to explain what is happening, although they might not be completely accurate?

mopc
2005-Feb-22, 07:28 PM
I see that gravity is quite tricky, huh? :-?

I never understood it. I mean, there are four basic forces in the universe: strong nuclear, weak nuclear, electromag and gravity.

But the first three I see as a consequence of how the atoms are formed. The nucleus needs two forces to exist and the electrons have their thing.

Gravity however seems to exist out of the blue, and like GR says, it's not a force but a property of space-time geometry affected by mass. But if it's not a force it cannot have a speed, I guess Einstein only thought of already-existing gravity fields and not what would happen if they changed. Maybe if he did we'd know C as the speed of gravity too, or better, as someone brilliantly wrote above, the Speed of Reality.

But if Quantum is right then a graviton woudnt need an entity called "space-time" to fly around. Then to Quantum physics space-time is the aether,and to Relativity the graviton is just imagination... or could both space-time and gravitons co-exist and be friends?

W.F. Tomba
2005-Feb-22, 08:08 PM
So, would you say that theorists use these kind of invented terms and hypothetical “models”, because they seem to work out, although they might not actually be a completely accurate explanation of what is happening, but later, hopefully, physicists will figure out what is actually happening, but right now they’ll use these tentative concepts because they seem to explain what is happening, although they might not be completely accurate?
"What is actually happening" may not be anything that can be imagined with reference to our everyday experience, so I'm not sure we can ever "figure it out" on an intuitive level. There doesn't have to be an underlying reality that seems natural to us.

But you are right that the terms and imagery used by physicists are just models chosen because they give the right answers. Furthermore, many of the concepts that seem "common sense" to us now are just old imaginary models that we have gotten used to. The idea that matter is made of atoms, for example, conflicts with our everyday experience of matter as continuous. The atomic theory was adopted because it gave the right answers, not because it fit with our macroscopic view of the world. It seems natural to us now only because it has pervaded our culture.

Space Monkey
2005-Feb-23, 12:15 AM
Ok, whats the proof that gravity travel at the speed of light?

If you had a bed the size of the solarsystem. Dropped a bowling ball with the mass of the sun in the middle. And had 9 cue balls with the mass of the corresponding planets. (take into effect that is is one of those memory foam mattresses that doesn't have any bounce)

The cue balls would be effected instantly. (I know we exsist in a 4 dimension world)

Not bowling ball falls, 8 minutes later... Earths cue ball is disturbed.

worzel
2005-Feb-23, 12:37 AM
I think that we shouldn't get too hung up over issues like whether gravity is due to the geometry of spacetime or particles, or what this or that German author meant by this or that word. The theories are mathematical - the words just help us to visualize these theories but the theories rest on the maths, not the wordy descriptions.

The fact that GR and QFT describe gravity differently is highly suggestive that the maths of both may well be subsumed by another theory that may well give rise to a comletely different description. Maybe Pythagoras was right all along and everything is harmonious music after all (guitar music that is).

worzel
2005-Feb-23, 12:48 AM
if you have a rigid rod a light-year long, won’t the force you transmit by pushing on it move faster than the speed of light?’ It turns out, no, it’ won’t—nothing is so rigid that it can transmit a compression wave faster than the speed of light.
But even if it were perfectly rigid (it's a thought experiment), it still couldn't be used to transmit a signal faster than c according to SR.

Sam5
2005-Feb-23, 02:54 AM
But you are right that the terms and imagery used by physicists are just models chosen because they give the right answers. Furthermore, many of the concepts that seem "common sense" to us now are just old imaginary models that we have gotten used to. The idea that matter is made of atoms, for example, conflicts with our everyday experience of matter as continuous. The atomic theory was adopted because it gave the right answers, not because it fit with our macroscopic view of the world. It seems natural to us now only because it has pervaded our culture.

Thanks for the information. It makes sense.

W.F. Tomba
2005-Feb-23, 03:31 AM
Ok, whats the proof that gravity travel at the speed of light?

If you had a bed the size of the solarsystem. Dropped a bowling ball with the mass of the sun in the middle. And had 9 cue balls with the mass of the corresponding planets. (take into effect that is is one of those memory foam mattresses that doesn't have any bounce)

The cue balls would be effected instantly. (I know we exsist in a 4 dimension world)

Not bowling ball falls, 8 minutes later... Earths cue ball is disturbed.
The wave that propagates through a mattress when you drop a bowling ball on it travels much slower than light. It would only seem instantaneous with a normal sized mattress, because the distance is so short. If your mattress were the size of a city block you might even see the wave traveling. So this analogy doesn't work at all.

Sam5
2005-Feb-23, 03:41 AM
The wave that propagates through a mattress when you drop a bowling ball on it travels much slower than light. It would only seem instantaneous with a normal sized mattress, because the distance is so short. If your mattress were the size of a city block you might even see the wave traveling. So this analogy doesn't work at all.

You are absolutely correct.

If you place some coins on your bed and drop a bowling ball on it, all the coins will appear to “instantly” jump up. However, this is only because your bed is small and the shock wave is moving through it rapidly.

Sound waves travel through air at about 1,100 feet per second, and that seems like “instantly” if they are coming to you from only 6 feet away (the length of a bed), but it seems much longer if they are coming to you from a mile away.

2005-Feb-23, 07:18 AM
dgavin
The way you get your duality out of gravity is to allow space-time to warp both ways. Near a massive object gravity warps space by erasing it, so that the distance between two objects is less in a second than it is right now. Far from distant objects you allow gravity to warp space-time in the opposite direction, adding it between objects.

I'm thinking that you've probably got both effects going on at the same time. Space is added everywhere, but it's a small amount according to the Hubble Mostly Constant. Space though only disappears near a massive object, so that's where you notice it.

If you buy into this idea, then you immediately have to ask how mass can be associated with expanding space. While thinking about this I wondered if mass came with its own space-time. If you lose mass then you lose space-time, a condition only near a massive object. If you gain mass then you also gain space-time, a condition that does not have to be associaed with massive objects. You could have a proton appear in empty space so long as about 100,000 cubic centimeters of space-time also appeared. One implication is that this allows you to violate the conservation of matter and energy because it's really the conservation of matter-energy and space-time. Both together would equal zero. Another implication is that empty space should gradually fill with elementary particles like protons and electrons, the building blocks of molecular hydrogen and thereafter new galaxies.

Grey
2005-Feb-23, 07:39 AM
Ok, I think that’s a pretty good explanation.
Thanks!

So, would you say that theorists use these kind of invented terms and hypothetical “models”, because they seem to work out, although they might not actually be a completely accurate explanation of what is happening, but later, hopefully, physicists will figure out what is actually happening, but right now they’ll use these tentative concepts because they seem to explain what is happening, although they might not be completely accurate?
Some people certainly think that quantum mechanics and the like are purely mathematical models which aren't necessarily related to reality. I tend to think that when a theory gets things right (especially to such a high degree of precision) it's usually a sign that the model used does reflect some element of reality well. As with W.F. Tomba's comment about atoms, atomic theory was accepted because it fit the observations, even though we couldn't see the actual atoms. Now, though, we can look at that small a scale and we can see that matter really is made of atoms. The model was right! Of course, you're also correct that these models are always at least somewhat tentative, and it's important to remember that our scientific view of the world is only the best one we have so far, and could potentially be shown to be wrong tomorrow. But even then, it could only be shown to be wrong up to a certain point, I think. I don't think anyone could come up with a reasonable theory that matter is continuous and not made of atoms, for example, so some theoretical models are less tentative than others.

Is gravity a curvature of spacetime or an exchange of particles? I'm not sure, but I expect in the end it will turn out to be the latter. But maybe worzel's right and there's a way to look at it that lets you realize that these two apparently different views are really just different ways of looking at the same thing.

swansont
2005-Feb-23, 12:50 PM
Hate to point this out, but there is no evidence yet that gravity effects are limited to 'C'.

If one of the reasons fo the gravity wave detectory. If Gravitatioanly waves can be detected then it would be contrained by 'GR'. If they arn't...

Until we know though you can't say that gravity is limited, or not limited to light speed.

It's still an unknown.

I don't think you can validly say there is no evidence. The study of a binary pulsar system, PSR 1913+16, shows a decay in orbit that is consistent with GR to a fairly good degree. The predicted orbit decay is from gravity waves propagating at c (or very close to it, within experimental error) - you need the retardation in order to exert a torque.

A central force would not be able to do that.

There has also been the investigation by Sergei Kopeikin. Though there is some question over the validity of his results, I found his argument compelling when he gave a colloquium here a few months ago, but then, I am not a GR expert.

worzel
2005-Feb-23, 12:54 PM
But maybe worzel's right and there's a way to look at it that lets you realize that these two apparently different views are really just different ways of looking at the same thing.
I hope you meant "worzel is right, maybe there's a way to look..." Subtle difference, but I'm not claiming I know how it's gonna work out.

Grey
2005-Feb-23, 07:06 PM
I hope you meant "worzel is right, maybe there's a way to look..." Subtle difference, but I'm not claiming I know how it's gonna work out.
Yes, that's what I meant. Apologies if there was any confusion.

John Dlugosz
2005-Feb-23, 09:28 PM
Thus the graviton is a quantization of the shadow cast by the mass. So gravity travels at the speed of dark.

Doesn't Feynman show that this can't be the case?

John Dlugosz
2005-Feb-23, 09:39 PM
As for no one measuring or detecting it, it's probably just a matter of time and more precise detectors.

Gravity Probe B, measurements in progress. Very delicate--don't touch it!

Grey
2005-Feb-23, 09:47 PM
Gravity Probe B, measurements in progress. Very delicate--don't touch it!
Gravity Probe B is a pretty amazing experiment, but I believe that it's looking for frame dragging effects as confirmation of general relativity, rather than gravitons or gravitational waves, which are being searched for with tools like LIGO.

John Dlugosz
2005-Feb-23, 09:52 PM
Is light emitted when this process takes place? Why don’t we just call it a “photon”?

See the physics faq. There is a copy at http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html and lots of other places.

My take on it:

Taking the mathematical description of this "thing", you might say that it closely resembles the formula of a photon, except that it has no momentum and other funny things. But all in all, it describes a photon more than it describes anything else...and when pondering whether it's a funny photon or a different class of things altogether, you realize that the funny values are all degenerate or pathalogical cases of the general formula for a photon. So that cinches it: it's a "virtual" photon, to pick an adjective.

--John

John Dlugosz
2005-Feb-23, 10:07 PM
Gravity Probe B, measurements in progress. Very delicate--don't touch it!
Gravity Probe B is a pretty amazing experiment, but I believe that it's looking for frame dragging effects as confirmation of general relativity, rather than gravitons or gravitational waves, which are being searched for with tools like LIGO.

Frame-dragging is part of the whole system that provides for the GR explaination of gravity. Also, the other effect being measured by GP.B is that space is smaller near the Earth. Space shrinking and expanding is what a gravity wave does as it passes. If the space near a massive object is sqeezed (which is being directly measured), then it follows that a moving object will create a moving disturbance, since your distance to it is changing and the amount of squeeze depends on distance. That is, a stationary instrument hoving as the Earth passed in its orbit would see the "dent" in spacetime come and then go again. BP.B is moving with the dent, and is measuring the details of the dent. This dent would be a wave to an observer in a different reference frame.

--John

Space Monkey
2005-Feb-24, 12:24 AM
Ok, whats the proof that gravity travel at the speed of light?

If you had a bed the size of the solarsystem. Dropped a bowling ball with the mass of the sun in the middle. And had 9 cue balls with the mass of the corresponding planets. (take into effect that is is one of those memory foam mattresses that doesn't have any bounce)

The cue balls would be effected instantly. (I know we exsist in a 4 dimension world)

Not bowling ball falls, 8 minutes later... Earths cue ball is disturbed.
The wave that propagates through a mattress when you drop a bowling ball on it travels much slower than light. It would only seem instantaneous with a normal sized mattress, because the distance is so short. If your mattress were the size of a city block you might even see the wave traveling. So this analogy doesn't work at all.

NO WAVES MAN!
It's a Sharper image mattress (ya know the kind that uses NASA technology) the size of our solar system.

If I can imagine a mattress the size of the solar system, I can give it waveless properties if I want to.

Heh!

You can't out imagine me! Especially with my own imagination.

Sam5
2005-Feb-24, 12:31 AM
My take on it:

Taking the mathematical description of this "thing", you might say that it closely resembles the formula of a photon, except that it has no momentum and other funny things. But all in all, it describes a photon more than it describes anything else...and when pondering whether it's a funny photon or a different class of things altogether, you realize that the funny values are all degenerate or pathalogical cases of the general formula for a photon. So that cinches it: it's a "virtual" photon, to pick an adjective.

--John

Ok, I’ll accept that. It’s a “funny” kind of photon. Maybe it’s there, maybe it’s not. But in the mean time, it seems like it is there and so we’ll pretend it is there, and we’ll call it a “virtual” photon. That’s ok with me.

I wish guys who write QM books would explain things this way. I can understand tentative ideas, hunches, suppositions, etc. Now QM is beginning to make a little sense to me.

W.F. Tomba
2005-Feb-24, 12:32 AM

NO WAVES MAN!
It's a Sharper image mattress (ya know the kind that uses NASA technology) the size of our solar system.
I know you meant a mattress with no "bounce." It doesn't matter. Any disturbance in the mattress, even just a depression, will travel outward as a wave.

Sam5
2005-Feb-24, 12:36 AM

NO WAVES MAN!
It's a Sharper image mattress (ya know the kind that uses NASA technology) the size of our solar system.
I know you meant a mattress with no "bounce." It doesn't matter. Any disturbance in the mattress, even just a depression, will travel outward as a wave.

Yep, even a Sharper Image mattress will have waves traveling within it. Especially if it is a water-bed mattress.

W.F. Tomba
2005-Feb-24, 12:36 AM
I wish guys who write QM books would explain things this way. I can understand tentative ideas, hunches, suppositions, etc. Now QM is beginning to make a little sense to me.
Just remember that while the physical description may be tentative, the math behind it isn't. So while you can certainly say that virtual photons may or may not be there, that doesn't mean the theory may or may not be mathematically accurate.

Sam5
2005-Feb-24, 01:01 AM
I wish guys who write QM books would explain things this way. I can understand tentative ideas, hunches, suppositions, etc. Now QM is beginning to make a little sense to me.
Just remember that while the physical description may be tentative, the math behind it isn't. So while you can certainly say that virtual photons may or may not be there, that doesn't mean the theory may or may not be mathematically accurate.

Ok.

Normandy6644
2005-Feb-24, 01:02 AM
I wish guys who write QM books would explain things this way. I can understand tentative ideas, hunches, suppositions, etc. Now QM is beginning to make a little sense to me.
Just remember that while the physical description may be tentative, the math behind it isn't. So while you can certainly say that virtual photons may or may not be there, that doesn't mean the theory may or may not be mathematically accurate.

The other thing is that a lot of the physics actually comes from the mathematical solutions in QM, which is somewhat counterintuitive. In Newtonian mechanics, the math was developed to explain the physics, but a lot of times in QM you get the math telling you what the physics has to be.

Sam5
2005-Feb-24, 01:22 AM
I wish guys who write QM books would explain things this way. I can understand tentative ideas, hunches, suppositions, etc. Now QM is beginning to make a little sense to me.
Just remember that while the physical description may be tentative, the math behind it isn't. So while you can certainly say that virtual photons may or may not be there, that doesn't mean the theory may or may not be mathematically accurate.

The other thing is that a lot of the physics actually comes from the mathematical solutions in QM, which is somewhat counterintuitive. In Newtonian mechanics, the math was developed to explain the physics, but a lot of times in QM you get the math telling you what the physics has to be.

So the math is never wrong, anytime, anywhere? It’s always right? Nobody ever makes any math mistakes?

Grey
2005-Feb-24, 01:41 AM
So while you can certainly say that virtual photons may or may not be there, that doesn't mean the theory may or may not be mathematically accurate.
And remember that in some sense, what the theory is that virtual photons are something that may or may not be there, depending on what you mean by "be". :D

So the math is never wrong, anytime, anywhere? It’s always right? Nobody ever makes any math mistakes?
Well, of course people do, but that's why other people check it over, and ultimately why we compare what the theory predicts with what we actually see. When they coincide, we assume we've gotten the math right.

Normandy6644
2005-Feb-24, 02:34 AM
I wish guys who write QM books would explain things this way. I can understand tentative ideas, hunches, suppositions, etc. Now QM is beginning to make a little sense to me.
Just remember that while the physical description may be tentative, the math behind it isn't. So while you can certainly say that virtual photons may or may not be there, that doesn't mean the theory may or may not be mathematically accurate.

The other thing is that a lot of the physics actually comes from the mathematical solutions in QM, which is somewhat counterintuitive. In Newtonian mechanics, the math was developed to explain the physics, but a lot of times in QM you get the math telling you what the physics has to be.

So the math is never wrong, anytime, anywhere? It’s always right? Nobody ever makes any math mistakes?

That's not really the point though. If a whole bunch of people solve an equation and it says something about what's happening to a system, then there is a very high probability (to stick with QM terms here :wink: ) that it's correct.

John Dlugosz
2005-Feb-24, 11:15 PM
So the math is never wrong, anytime, anywhere? It’s always right? Nobody ever makes any math mistakes?

Individual people make math errors, but real scientists have the good sence to get a second opinion. Such errors are found before the results are published. In fact, proper independant working out of the problem is a requirement before a reputable journal will publish it.

As for the math (model) being wrong, we have a better understanding now that we have to track the limits and ranges as well, and those are verified against observations.

For example, Newton's physics assumes that the formulas are always true. Today we know that it's accurate only for low velocities. Today, we think of these things and realize that extrapolation is risky, and keep track of the most extreme observed examples so we know when we're getting off our known turf.

Finding places where the numbers come up wrong is what exploration is all about! The worst nightmare of physicists is that the results of running the new LHC will be exactly what they expected. Things "break" along the unexplored edges, not in the middle! The standard model is the most successful explaination of phenomona, EVER.

When the new collider smashes together large objects at energies higher than anything tried before, it will not be a matter of being "wrong" about what they predicted, but how many decimal places agree with the prediction before there is a very slight difference hinting at something new that never showed up before.

John Dlugosz
2005-Feb-24, 11:23 PM
Just remember that while the physical description may be tentative, the math behind it isn't. So while you can certainly say that virtual photons may or may not be there, that doesn't mean the theory may or may not be mathematically accurate.

There are different ways of modeling things that give the same (correct, observed) result.

Using Maxwell's equasions, you say that light is made up of electric and magnetic fields. The ability to jump across a small gap is due to "evencent waves" caused by the electric field component extending past the edge.

Using QM, you say that electric and magnetic fields are caused by virtual photons. That's exactly the other way around! Which is the more fundimantal? The ability to cross the gap is due to uncertainty and tunneling. But it still crosses the gap.

The former model still works fine for working with radio, and trying to crank through the complex QM is just more work, and detracts from developing an intuitive understanding of transmitters and receivers. But, the model breaks down at short distances and/or high energies.

Bathcat
2005-Feb-27, 07:29 PM
From the reading I've done it seems that at least some physicists take the view that whether spacetime "really" bends or whether spacetime is "really" flat and gravity distorts clocks and rulers is unimportant because, as far as a mathematical description of gravitation goes, the approaches give identical results.

Sometimes some problems are more tractable using one method of casting the equations; sometimes the other seems more effective.

Just as sometimes it's best to calculate the actions of light as a wave phenomenon; sometimes as a particle. Both approaches work. What the photon "really IS" (if we ever figure out Ultimate Reality) will not falsify either way of calculating the behavior of light.

I think the same will be true for a quantum theory of gravitation: the model of spacetime curvature will stay in physicists' toolbox because it is an excellent way of calculating the behavior of gravity for many, many situations. Whether spacetime curvature is the "Ultimate Reality" is almost irrelevant...almost, but perhaps, as Grey notes, not quite!

---

Someone -- I apologize for not noting who it was -- wrote that sometimes gravity obeys the lightspeed limit and under other conditions it doesn't.

I believe that is incorrect. A GR specialist once informed me, somewhat curtly, that the Earth responds to the Sun's gravitational field NOT because the field travels instaneously from Sun to Earth, but because the field is already present at Earth and has already been shaped by the Sun. The field at Earth was given its shape by the Sun 8 minutes previously, if you want to look at it that way.

As far as we can tell, gravitation never propagates at superluminal speeds. There is no evidence for such propagation, and nothing in the standard model of physical law predicts such superluminal propagation.

---

On yet another comment: Yes, gravity gravitates! My layman's understanding is that a sufficiently strong gravitational field produces a component of self-gravitation: the field itself adds to its own strength. The effect only becomes significant in very strong fields, ie near black hole singularities. At least that's what I seem to remember.

Grey can probably explain this much better than old and sieve-memoried Bathcat, though... :)

junkyardfrog
2005-Feb-28, 07:32 AM
Speed of gravity:

http://www.ldolphin.org/vanFlandern/gravityspeed.html

junkyardfrog
2005-Feb-28, 08:26 AM
P.S. Where is BA on this one? I'd like to hear what he has to say.

swansont
2005-Feb-28, 12:34 PM
Speed of gravity:

http://www.ldolphin.org/vanFlandern/gravityspeed.html

Yeah, but it's van Flandern. How does one assess the merit of the arguments? Is it real, or is it crackpottery?

Tensor
2005-Feb-28, 12:52 PM
On yet another comment: Yes, gravity gravitates! My layman's understanding is that a sufficiently strong gravitational field produces a component of self-gravitation: the field itself adds to its own strength.

Depends on what you mean by sufficiently strong. Gravity (or space-time curvature) is tied to the stress-energy tensor. Since a gravity field has an energy of it's own, that energy also contributes to the field. The equations of GR are very non-linear, and this is a second order effect.

The effect only becomes significant in very strong fields, ie near black hole singularities. At least that's what I seem to remember.

Not quite, even the sun produces a measurable effect. The precession of Mercury is explained by this second order effect.

Grey can probably explain this much better than old and sieve-memoried Bathcat, though... :)

Actually, you weren't that far off.

Bathcat
2005-Feb-28, 05:24 PM
I think that VanFlandern's superluminal gravitation hypothesis has been shown to be flatly mistaken. It is disregarded by physicists because the effects VanFlandern notes are very well understood in the context of GR, and furthermore VanFlandern's hypothesis is not only unnecessary but is contradicted by what we understand of physical law.

There is, or was, a physics paper somewhere online explaining in detail why VanFlandern is full of beans on this one. The paper was written by Steve Carlip. I will go a-Googling in a bit and see if I can track it down.

Heh, I did ask a physicist about VF's superluminal gravitation once. He was a bit testy about it: Why are you wasting your time with garbage like that? he wanted to know. :)

junkyardfrog
2005-Mar-01, 04:17 AM
Yeah....

I know....

It is VF. But I still would like to hear what BA has to say about this:

Indeed, it is widely accepted, even if less widely known, that the speed of gravity in Newton's Universal Law is unconditionally infinite. (e.g., Misner et al., 1973, p.177) This is usually not mentioned in proximity to the statement that GR reduces to Newtonian gravity in the low-velocity, weak-field limit because of the obvious question it begs about how that can be true if the propagation speed in one model is the speed of light, and in the other model it is infinite.

The same dilemma comes up in many guises: Why do photons from the Sun travel in directions that are not parallel to the direction of Earth's gravitational acceleration toward the Sun?

AT
2005-Mar-01, 04:22 AM
That statement is true; but only in Newtonian situations.
EDIT: What I mean is that Newton doesn't care what the speed of gravity is between objects 20 feet away. Its might as well be v=infinity.

junkyardfrog
2005-Mar-01, 04:25 AM
That statement is true; but only in Newtonian situations.
EDIT: What I mean is that Newton doesn't care what the speed of gravity is between objects 20 feet away. Its might as well be v=infinity.

Well....

Correct me if I'm wrong, but isn't that about 99.99% of all "situations"?

junkyardfrog
2005-Mar-01, 04:27 AM
EDIT: What I mean is that Newton doesn't care what the speed of gravity is between objects 20 feet away. Its might as well be v=infinity.

Arg....

Now you edit.

;)

Frankly I don't think anyone cares about the speed of gravity between objects that are 20 feet away!

junkyardfrog
2005-Mar-02, 04:10 AM
Sheesh!

Whatta I hafta do to get BA's attention on this thread?

BEG???

;)

mopc
2005-Mar-02, 04:38 AM
Sheesh!

Whatta I hafta do to get BA's attention on this thread?

BEG???

;)

Yeah, I think we need the intervention of a Higher Power here. It appears that science has no clue as to what gravity really is let alone how fast it propagates. It appears that people assume c because it's the limit anyway. What would the holy Stephen Hawkin answer here? Has he ever addressed the subject specifically? What about other top-notch scientists?

Grey
2005-Mar-02, 05:57 AM
Yeah, I think we need the intervention of a Higher Power here. It appears that science has no clue as to what gravity really is let alone how fast it propagates. It appears that people assume c because it's the limit anyway. What would the holy Stephen Hawkin answer here? Has he ever addressed the subject specifically? What about other top-notch scientists?
Under either general relativity or the properties we know a quantum theory of gravity needs to have, gravity propagates at the same speed as light. It's not just an arbitrary assumption; these are our best models of gravity at this point, and they do an impressive job of agreeing with observations.

As for Van Flandern, I'm not sure he really understands the theories he's criticizing. If you'd like to ask about some of his specific points, I can address them, but you might also just use the board's search function to dig up some previous threads about him.

Of course, if you'd really like the BA's input, I'd assume that he can't possibly look in on every thread here. Send him a private message with a link to the thread and ask if he'd be willing to comment.

swansont
2005-Mar-02, 01:05 PM
Sheesh!

Whatta I hafta do to get BA's attention on this thread?

BEG???

;)

Yeah, I think we need the intervention of a Higher Power here. It appears that science has no clue as to what gravity really is let alone how fast it propagates. It appears that people assume c because it's the limit anyway. What would the holy Stephen Hawkin answer here? Has he ever addressed the subject specifically? What about other top-notch scientists?

As I pointed out before, there have been experiments and observations that agree with the speed being c, as theory predicts, so "no clue" really isn't accurate.

Sam5
2005-Mar-02, 01:27 PM
As I pointed out before, there have been experiments and observations that agree with the speed being c, as theory predicts, so "no clue" really isn't accurate.

When you make a statement like that, I think it would be a good idea for you to list the specific experiments and observations. Why are people still searching for gravity waves and debating the speed of gravity propagation if the information is already known?

mopc
2005-Mar-02, 02:38 PM
From quick googling, I learned that the speed of gravity was first measured very recently, January 2003,

http://www.newscientist.com/article.ns?id=dn3232
http://www.newscientist.com/article.ns?id=dn2763

and the results were c. Many scientists disagree with these results:

"The claim that they've measured the speed of gravity is simply incorrect," said Clifford Will, a physicist at Washington University in St Louis, Missouri, and an expert in the field.
http://www.space.com/scienceastronomy/gravity_speed_030116.html

I mean, when I said "no clue" I really meant "a lot of doubts".

Grey
2005-Mar-02, 03:04 PM
I mean, when I said "no clue" I really meant "a lot of doubts".
Yes and no. There is some criticism over whether their observation really measures the speed at which gravity propagates. However, I don't think any mainstream physicists seriously doubt that gravity does propagate at the same speed as light. Just because we think something is true doesn't mean we don't keep trying to test it experimentally.

The same is true of Sam5's comment. We're trying to detect gravitational waves because we think our tools may finally be sensitive enough to do so, but we know that it won't be easy (the biggest problem is that any apparatus sensitive enough to detect them will also pick up the truck driving down the road a few miles away, so you have to come up with a method to screen out all the noise). Even though most physicists fully expect that they'll be eventually found doesn't mean that there's no reason to try to actually do so.

Heck, people have still been testing whether gravity follows an inverse square as equipment gets better and better. With the advent of string theory, there's some speculation that it might actually deviate from that, but even before string theory, people were running such experiments. They always expected to find that, yes, gravity follows an inverse square law and now we just know that to one more decimal place, but they still keep running the experiments just to see if there might be something anomalous.

Bathcat
2005-Mar-02, 07:01 PM
Newton rather famously refused to speculate on the propagation or cause of the gravitation he described.

I personally think that van Flandern is putting up a smoke screen when he talks about Newton's gravitation being instantaneous: yes, it was, but that's because Newton had no idea how gravitation was produced and how it propagated and Newton said as much himself.

General relativity does describe gravitation's "cause" (spacetime curvature or distortion of clocks and rulers, depending on how you squinch your eyes) and GR's description is perfectly valid in the Newtonian limit as well as outside it.

I think that what van Flandern tried to imply in the passage quoted above is that because Newtonian gravitation is instantaneous, and because Newtonian gravitation is sufficient for many real-world calculations, therefore instantaneous gravitational propagation really exists in the real world.

It's like saying that because Newton used absolute time and flat space, and we use Newtonian gravitation for real-world calculations, therefore absolute time and flat space really are the real-world case...and relativity is not.

I don't see that being a tenable proposition.

----

Incidentally, if one wants to appeal to a higher authority than Grey (whom I respect quite a lot) one can always email some physicists. I recommend that such emails be polite and concise, and I would email several as it's quite common that these gentlemen have a lot to do and not much time to answer random queries from the public.

Grey
2005-Mar-02, 07:31 PM
Incidentally, if one wants to appeal to a higher authority than Grey (whom I respect quite a lot) one can always email some physicists. I recommend that such emails be polite and concise, and I would email several as it's quite common that these gentlemen have a lot to do and not much time to answer random queries from the public.

Fortis
2005-Mar-03, 01:52 AM
Incidentally, if one wants to appeal to a higher authority than Grey (whom I respect quite a lot) one can always email some physicists. I recommend that such emails be polite and concise, and I would email several as it's quite common that these gentlemen have a lot to do and not much time to answer random queries from the public.
And whatever you do, don't use green or purple ink, or strange capitalisation. I knew one guy in an astronomy department who used to receive messages from some strange cult that believed that he was some sort of prophet. (He'd discovered some structure in the Orion Nebula that they believed to be God. :o ) ;)

mopc
2005-Mar-20, 07:53 AM
prophet..... weird comparison :o

frogesque
2005-Mar-20, 11:42 AM
... Finally, try to make the question a genuine request for information rather than espousing your own improved "theory" of the way the universe works. That is, don't start by saying "Einstein was completely wrong and here's why, now try to prove me wrong." :D

Care to repeat that on the 'Huygens' thread? 8)

Grey
2005-Mar-20, 04:32 PM
Care to repeat that on the 'Huygens' thread? 8)
I'm not sure that I do. :D It looks like others are making the case for rationality well over there, and I'm not sure I want to get sucked into that debate. :wink:

mopc
2005-May-16, 01:30 AM
Found this http://www.metaresearch.org/cosmology/gravity/speed_limit.asp

and remembered this old thread... any news on the subject?

Grey
2005-May-16, 02:03 PM
...and remembered this old thread... any news on the subject?
I don't think there's any new information here. Van Flandern's still mistaken, our best theories still state that the speed of gravity is the same as the speed of light, and the experimental evidence is consistent with that. The experimental constraint isn't incredibly strong, given the difficulty of measuring the effect, and I don't know of any knew evidence in the last few months.

gopher65
2005-May-17, 02:56 AM
I think it would be cool if the speed of gravity were infinite :D ! But unfortunately everything I've read suggests that it isn't :( . And the recent (well, fairly recent) observations of the two neutron stars locked in a tight orbit just adds yet another Olympic swimming pool of support to an already (nearly) full evidence bucket hehe