Is gravitation faster than light?

But suppose gravity is itself caused by waves? Since it is generally admitted that we have no satisfactory theory as to the cause of gravity, perhaps my own model deserves consideration. It is possibly similar in effect to string theory, but involves no "gravitons", whether real or virtual, only waves, which are being produced and absorbed all the time by all condensed matter. Under my "Phi-Wave Aether" model, gravity is of the same nature as all other forces: they all travel at speed c as waves in the aether, the differences between them being due to different degrees of coherence and different higher-level periodic patterns superposed on very high frequency longitudinal waves. The "fields" are themselves formed by waves, and are all constantly updated, whether or not the sources are moving.

Just a thought ...

I haven't tried to incorporate black holes into the theory, but if I'm right there is no avoiding propagation at speed c.

Caroline
http://freespace.virgin.net/ch.thompson1/

The variations in the gravity field caused by linear motion will of course result in a continuous "update" of the gravity field of an object, but they do not create "waves" of gravity. To create a wave from any motion requires an oscillation; this is a general fact of physics, not limited to gravity fields but also true (as my example showed) of the electromagnetic field. You cannot use the analogy of water waves, like from the prow of a boat, which is what it sounds like you are trying to do; water waves are transverse waves, but light and gravity waves are longitudinal, and furthermore while water presents resistance to the movement of objects, space does not.

In general relativity, on the other hand, gravity propagates at the speed of light; that is, the motion of a massive object creates a distortion in the curvature of spacetime that moves outward at light speed. This might seem to contradict the Solar System observations described above, but remember that general relativity is conceptually very different from Newtonian gravity, so a direct comparison is not so simple. Strictly speaking, gravity is not a "force" in general relativity, and a description in terms of speed and direction can be tricky. For weak fields, though, one can describe the theory in a sort of Newtonian language. In that case, one finds that the "force" in GR is not quite central--it does not point directly towards the source of the gravitational field--and that it depends on velocity as well as position. The net result is that the effect of propagation delay is almost exactly cancelled, and general relativity very nearly reproduces the Newtonian result.

This cancellation may seem less strange if one notes that a similar effect occurs in electromagnetism. If a charged particle is moving at a constant velocity, it exerts a force that points toward its present position, not its retarded position, even though electromagnetic interactions certainly move at the speed of light. Here, as in general relativity, subtleties in the nature of the interaction "conspire" to disguise the effect of propagation delay. It should be emphasized that in both electromagnetism and general relativity, this effect is not put in ad hoc but comes out of the equations. Also, the cancellation is nearly exact only for constant velocities. If a charged particle or a gravitating mass suddenly accelerates, the change in the electric or gravitational field propagates outward at the speed of light.

Since this point can be confusing, it's worth exploring a little further, in a slightly more technical manner. Consider two bodies--call them A and B--held in orbit by either electrical or gravitational attraction. As long as the force on A points directly towards B and vice versa, a stable orbit is possible. If the force on A points instead towards the retarded (propagation-time-delayed) position of B, on the other hand, the effect is to add a new component of force in the direction of A's motion, causing instability of the orbit. This instability, in turn, leads to a change in the mechanical angular momentum of the A-B system. But total angular momentum is conserved, so this change can only occur if some of the angular momentum of the A-B system is carried away by electromagnetic or gravitational radiation.

Now, in electrodynamics, a charge moving at a constant velocity does not radiate. (Technically, the lowest order radiation is dipole radiation, which depends on the acceleration.) So, to the extent that A's motion can be approximated as motion at a constant velocity, A cannot lose angular momentum. For the theory to be consistent, there must therefore be compensating terms that partially cancel the instability of the orbit caused by retardation. This is exactly what happens; a calculation shows that the force on A points not towards B's retarded position, but towards B's "linearly extrapolated" retarded position. Similarly, in general relativity, a mass moving at a constant acceleration does not radiate (the lowest order radiation is quadrupole), so for consistency, an even more complete cancellation of the effect of retardation must occur. This is exactly what one finds when one solves the equations of motion in general relativity.

What happens physically, if real gravitons of a gravitational wave are kept inside a black hole? Will they come across the same point in space more than once? Will they thus curve space stronger? Will they accumulate behind the event horizon? Will they fall back into singularity? Will the gravitational waves interfere with each other?

I know, I am still curious…

Carsten

Hee hee, nobody knows the answers to any of these questions. Remember Hawking: "A black hole has no hair."

D.09 How can gravity escape from a black hole?

In a classical point of view, this question is based on an incorrect
picture of gravity. Gravity is just the manifestation of spacetime
curvature, and a black hole is just a certain very steep puckering
that captures anything that comes too closely. Ripples in the
curvature travel along in small undulatory packs (radiation---see
D.05), but these are an optional addition to the gravitation that is
already around. In particular, black holes don't need to radiate to
have the fields that they do. Once formed, they and their gravity
just are.

In a quantum point of view, though, it's a good question. We don't
yet have a good quantum theory of gravity, and it's risky to predict
what such a theory will look like. But we do have a good theory of
quantum electrodynamics, so let's ask the same question for a charged
black hole: how can a such an object attract or repel other charged
objects if photons can't escape from the event horizon?

The key point is that electromagnetic interactions (and gravity, if
quantum gravity ends up looking like quantum electrodynamics) are
mediated by the exchange of *virtual* particles. This allows a
standard loophole: virtual particles can pretty much "do" whatever they
like, including travelling faster than light, so long as they disappear
before they violate the Heisenberg uncertainty principle.

The black hole event horizon is where normal matter (and forces) must
exceed the speed of light in order to escape, and thus are trapped.
The horizon is meaningless to a virtual particle with enough speed.
In particular, a charged black hole is a source of virtual photons
that can then do their usual virtual business with the rest of the
universe. Once again, we don't know for sure that quantum gravity
will have a description in terms of gravitons, but if it does, the
same loophole will apply---gravitational attraction will be mediated
by virtual gravitons, which are free to ignore a black hole event
horizon.

See R Feynman QED (Princeton, ???) for the best nontechnical account
of how virtual photon exchange manifests itself as long range
electrical forces.

In a few words - if that's possible - how does your idea differ from GR?

In a few words - if that's possible - how does your idea differ from GR?

(a) It is not mathematical, only intuitive.
(b) It provides an idea for actual cause for gravity.
(c) It quite unashamedly assumes an aether, not just letting one creep in by the back door.

Of couse it is not much use as a "theory" because of its lack of equations, but it does make a few qualitative predictions. For more see my web site. I have been cautioned not to try and introduce personal theories on this forum, otherwise I'd say more here.

Once again, we don't know for sure that quantum gravity
will have a description in terms of gravitons, but if it does, the
same loophole will apply---gravitational attraction will be mediated
by virtual gravitons, which are free to ignore a black hole event
horizon.

See R Feynman QED (Princeton, ???) for the best nontechnical account
of how virtual photon exchange manifests itself as long range
electrical forces.

Well, thank you all for your posts!

I will no be able to answer quickly to all of them, but here is the most important one for me:



So this leads back to post # 1:

At least virtual gravitons are able to move faster than c to escape the event horizon.

Is this also true for virtual photons in an EM field?

That implies, that information can leave a black hole, correct?

Carsten

Do they go faster than light? Do virtual particles contradict relativity or causality?

In section 2, the virtual photon's plane wave is seemingly created everywhere in space at once, and destroyed all at once. Therefore, the interaction can happen no matter how far the interacting particles are from each other. Quantum field theory is supposed to properly apply special relativity to quantum mechanics. Yet here we have something that, at least at first glance, isn't supposed to be possible in special relativity: the virtual photon can go from one interacting particle to the other faster than light! It turns out, if we sum up all possible momenta, that the amplitude for transmission drops as the virtual particle's final position gets further and further outside the light cone, but that's small consolation. This "superluminal" propagation had better not transmit any information if we are to retain the principle of causality.

I'll give a plausibility argument that it doesn't in the context of a thought experiment. Let's try to send information faster than light with a virtual particle.

Suppose that you and I make repeated measurements of a quantum field at distant locations. The electromagnetic field is sort of a complicated thing, so I'll use the example of a field with just one component, and call it F. To make things even simpler, we'll assume that there are no "charged" sources of the F field or real F particles initially. This means that our F measurements should fluctuate quantum- mechanically around an average value of zero. You measure F (really, an average value of F over some small region) at one place, and I measure it a little while later at a place far away. We do this over and over, and wait a long time between the repetitions, just to be safe.



After a large number of repeated field measurements we compare notes. We discover that our results are not independent; the F values are correlated with each other-- even though each individual set of measurements just fluctuates around zero, the fluctuations are not completely independent. This is because of the propagation of virtual quanta of the F field, represented by the diagonal lines. It happens even if the virtual particle has to go faster than light.

However, this correlation transmits no information. Neither of us has any control over the results we get, and each set of results looks completely random until we compare notes (this is just like the resolution of the famous EPR "paradox").

You can do things to fields other than measure them. Might you still be able to send a signal? Suppose that you attempt, by some series of actions, to send information to me by means of the virtual particle. If we look at this from the perspective of someone moving to the right at a high enough speed, special relativity says that in that reference frame, the effect is going the other way:


Now it seems as if I'm affecting what happens to you rather than the other way around. (If the quanta of the F field are not the same as their antiparticles, then the transmission of a virtual F particle from you to me now looks like the transmission of its antiparticle from me to you.) If all this is to fit properly into special relativity, then it shouldn't matter which of these processes "really" happened; the two descriptions should be equally valid.

We know that all of this was derived from quantum mechanics, using perturbation theory. In quantum mechanics, the future quantum state of a system can be derived by applying the rules for time evolution to its present quantum state. No measurement I make when I "receive" the particle can tell me whether you've "sent" it or not, because in one frame that hasn't happened yet! Since my present state must be derivable from past events, if I have your message, I must have gotten it by other means. The virtual particle didn't "transmit" any information that I didn't have already; it is useless as a means of faster-than-light communication.

The order of events does not vary in different frames if the transmission is at the speed of light or slower. Then, the use of virtual particles as a communication channel is completely consistent with quantum mechanics and relativity. That's fortunate: since all particle interactions occur over a finite time interval, in a sense all particles are virtual to some extent.

-----------------------------------------------
3b. How meaningful are single Feynman diagrams?
-----------------------------------------------

The standard model is a theory defined in terms of a Lagrangian.
To get computable output, Feynman graph techniques are used.
But individual Feynman graphs are meaningless (often infinite);
only the sum of all terms of a given order can be given - after
a process called renormalization - a well-defined (finite) meaning.
This is well-known; so no-one treats the Feynman graphs as real.
What is taken as real is the final outcome of the calculations,
which can be compared with measurements.

-------------------------------------
3c. How real are 'virtual particles'?
-------------------------------------

All language is only an approximation to reality, which simply is.
But to do science we need to classify the aspects of reality
that appear to have more permanence, and consider them as real.
Nevertheless, all concepts, including 'real' have a fuzziness
about them, unless they are phrased in terms of rigorous mathematical
models (in which case they don't apply to reality itself but only to
a model of reality).

In the informal way I use the notion, 'real' in theoretical physics
means a concept or object that
- is independent of the computational scheme used to
extract information from a theory,
- has a reasonably well-defined and consistent formal basis
- does not give rise to misleading intuition.
This does not give a clear definition of real, of course.
But it makes for example charge distributions, inputs and outputs of
(theoretical models of) scattering experiments, and quarks something
real, while making bare particles and virtual particles artifacts of
perturbation theory.

Quarks must be considered real because one cannot dispense with them
in any coherent explanation of high energy physics.

Virtual particles must not be considered real since they arise only in
a particular approach to high energy physics - perturbation theory
before renormalization - that does not even survive the modifications
needed to remove the infinities. Moreover, the virtual particle content
of a real state depends so much on the details of the computational
scheme (canonical or light front quantization, standard or
renormalization group enhances perturbation theory, etc.) that
calling virtual particles real would produce a very weird picture of
reality.

...

The figurative virtual objects in QFT are there only because of the
well-known limitations of the foundations of QFT. In a nonperturbative
setting they wouldn't occur at all. This can be seen by comparing with
QM. One could also do nonrelativistic QM with virtual objects but
no one does so (except sometimes in motivations for QFT),
because it does not add value to a well-understood theory.

Virtual particles are an artifact of perturbation theory that
give an intuitive (but if taken too far, misleading) interpretation
for Feynman diagrams. More precisely, a virtual photon, say,
is an internal photon line in one of the Feynman diagrams. But there
is nothing real associated with it. Detectable photons are always
real, 'dressed' photons.

Virtual particles, and the Feynman diagrams they appear in,
are just a visual tool of keeping track of the different terms
in a formal expansion of scattering amplitudes into multi-dimensional
integrals involving multiple propaators - the momenta of the virtual
particles represent the integration variables.
They have no meaning at all outside these integrals.
They get out of mathematical existence once one changes the
formula for computing a scattering amplitude.

Therefore virtual particles are essentially analogous to virtual
integers k obtained by computing
log(1-x) = sum_k x^k/k
by expansion into a Taylor series. Since we can compute the
logarithm in many other ways, it is ridiculous to attach to
k any intrinsic meaning. But ...

... in QFT, we have no good ways to compute scattering amplitudes
without at least some form of expansion (unless we only use the
lowest order of some approximation method), which makes
virtual particles look a little more real. But the analogy
to the Taylor series shows that it's best not to look at them
that way. (For a very informal view of QED in terms of clouds of
virtual particles see
http://groups.google.com/groups?sel...%40univie.ac.at
and the later mails in this thread.)

A sign of the irreality of virtual particles is the fact that
when one does partial resummations of diagrams (which is essential for
renormalization), many of the virtual particles disappear.
A fully nonperturbative theory would sum everything, and no virtual
particles would be present anymore. Thus virtual particles are
entirely a consequence of looking at QFT in a perturbative way
rather than nonperturbatively.

Caroline, all waves have energy, and take energy to make; they are all made by oscillation. They dissipate this energy into the environment continuously. Fields, on the other hand, represent static energy; there is a difference in the energy of the vacuum with the field and without the field; but fields do not disspate energy. It doesn't matter whether you are talking classically or in QM terms. Thus, your idea would violate mass/energy conservation.

The waves from which forces are formed are the self-same waves in the aether that, when carrying a different modulation, form radiation.

Radiation is, I understand, a wave that does not dissipate in a vacuum. I don't see the problem.

Incidentally, surely light does dissipate just a tiny bit, otherwise the night sky would be bright (Olbers' paradox)?

What experimental evidence do we have that forces don't dissipate similarly over similar distances?

In my Phi-Wave-Aether theory, the high-level patterns get smudged out so that radiation and the forces lose their effectiveness. The net intensity of the phi-waves, though, almost certainly stays the same. On the scale of the whole universe, there is conservation of "phi-energy".

Forces aren't formed from waves. A force is the action of a field; and a field is apparently a basic entity

Radiation is a wave made up of the variation of the field of the vacuum, which is all fields at their minimum potential in the absence of any field associated with an object, and that minimum potential plus the potential of the object's field in the presence of a field associated with an object.

In regard to Olbers' paradox, there are several different possible solutions, and the most widely accepted one is that the universe is not infinite in time.

If a gravitational wave curves space and space curvature is a gravitational field, isn´t a gravitational field a gravitational force? Thus the wave would have formed the force…

Yes. And there is dark matter absorbing light. And moreover, the universe might not be infinite in space.

There have been several good discussions on gravitation and its speed in the Special & General Relativity section of PF; do readers feel this thread should be moved there (it'd get the attention of folk who are very familiar with GR).

Forces aren't formed from waves. A force is the action of a field; and a field is apparently a basic entity ....

I'm sorry, I have not studied your proposal enough to comment on it.

I know this is what is generally thought, but I think it is holding back understanding. The assumption that there are no underlying waves -- that the field is an intrisically static phenomenon -- held back Einstein and Lorentz, preventing them from seeing how to link macroscopic forces with "quantum-level" ones.

At the quantum level, we know that everything is constantly changing. We know that "interference" phenomena are important. To me, this implies that at that level there are high-frequency waves, underlying all the forces and causing everything that we see.

The short answer is:NO.What do you mean,"gravitation can (escape a blackhole)"...???Gravitation IS a black hole (too)...A singularity in the gravitational field,that is...If u want to,a particular solution to the Einstein equations...

Daniel.