out on a limb without a wave to hop on

Odysseus
Odysseus
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AIUI (which is admittedly not

AIUI (which is admittedly not well) on its own general relativity doesn’t need gravitons, because it doesn’t even regard gravity as a force. Instead it’s considered a pseudoforce like “centrifugal forceâ€, an artefact of a coördinate system that’s ill-suited to the situation. In an appropriate non-Euclidean geometry, the conic-section curves traced by objects in free fall become geodesics, so no additional fields or messenger-particles appear in a description of gravitational effects from such a frame of reference.

I believe gravitons are part of one of the approaches that have been taken towards a unified theory, seeking to fit gravity into a similar framework to the other “fundamental forcesâ€, in particular one that works on small (quantum) scales.

Mike Hewson
Mike Hewson
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RE: .... on its own general

Quote:
.... on its own general relativity doesn’t need gravitons ....


Great ! That is so right. Can I use that ? :-)

Well put. GR doesn't need gravitons. It's fine thanks as it is. We only reasonably want a 'unified field' theory to apply to Big Bangs and black hole centers, those being uninteresting anyway in a predictive sense.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

merle van osdol
merle van osdol
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If centrifugal force and

If centrifugal force and gravity are pseudoforces. Is magnetism also a pseudoforce? It doesn't have a particle of its own; just tags along with the electron. It seems more like a 'field' like gravity.

What other forces may not be forces but be pseudoforces?

merle

What is freedom of expression? Without the freedom to offend, it ceases to exist.

— Salman Rushdie

Odysseus
Odysseus
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RE: If centrifugal force

Quote:
If centrifugal force and gravity are pseudoforces. Is magnetism also a pseudoforce? It doesn't have a particle of its own; just tags along with the electron. It seems more like a 'field' like gravity.


I believe electricity and magnetism are two manifestations of the same fundamental force and are always found together, each one acting in a normal direction to the other—respectively a push/pull and a twist, so to speak. Charge isn‘t supposed to distort spacetime the way mass does, and it’s harder to ignore photons than gravitons, so I can’t see magnetism as a pseudoforce.

Quote:
What other forces may not be forces but be pseudoforces?


I‘m probably out of my depth, but I‘d say anything that disappears when a suitable inertial frame of reference is adopted, one in which an affected object experiences no acceleration. The elevator Einstein used to illustrate the equivalence principle is a famous example. The introduction to the Wikipedia article mentions four classical types: reaction to linear acceleration, centrifugal force, Coriolis force, and Euler force.

Mike Hewson
Mike Hewson
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RE: If centrifugal force

Quote:
If centrifugal force and gravity are pseudoforces. Is magnetism also a pseudoforce?


No, not in the sense as discussed. But it is relativistic though. For a given specific charge one can go to that charged particle's rest frame and the magnetism from that charge goes away. This was raised by Einstein, addressed at the beginning of his famous paper in 1905 ( the topic of which we now call Special Relativity ). If you will suffer a spot of mathematics :

Quote:
F = q * [ E + (V x B) ]


This is a vector equation. Mr Lorentz was the one to describe this. F on the left here is the force on a specific particle with constant charge q ( a scalar or pure number ), with that particle having velocity V at a given moment. That force at some place and time has two parts to be vectorially added ( the purple plus ) :

- the first depending on the direction and strength of the electric field E

- the second depending on the direction and strength of the magnetic field B but taking into account the momentary velocity V with respect to that field.

The x indicates a special type of vector combination/operation, called the cross product, that mathematically manages the directions properly. In brief : V x B is perpendicular to both V and B with magnitude 'modulated' by the sine of the angle b/w them. In any event the total Lorentz Force, as it is called, is proportional to the charge q ( thus F would be zero for an electrically neutral body ).

(A) If you take the case of a magnet moving toward some conducting wire, the electrons in the wire we will consider at rest. So the V will be zero. At their position in the metallic lattice of the wire the magnetic field B due to the magnet will be changing because the magnet is moving. There is another law ( one of Mr Maxwell's ) that says an electric field is hence produced because of that changing magnetic field. Hence the Lorentz force above is :

Quote:
F = q * [ E + (0 x B) ] = q * E


this will push on the electron and a current in the conductor will result. So a magnet moving through a loop of wire will result in a current going around the loop, say. You could measure that current.

[ You may be familiar with such induction loops embedded in roadways near intersections for instance, so that when a car comes along ( a big lump of magnetic metal ) a current sensor in that loop says "aha! a car has arrived" and input that finding into a control system for the traffic lights. This annoys the crap out of cyclists who typically have much smaller aluminium ( non-magnetic ) bicycle frames. You can buy magnets to clip on to the bike though .... :-) ]

(B) If you take the case of a magnet sitting at rest but with a conducting wire moving nearby. In the sense of the setup of the problem the magnet produces no electric field ( the magnet has nett charge of zero ) and so now we have :

Quote:
F = q * [ 0 + (V x B) ] = q * V x B


and this force can now push the electrons in the wires metal lattice around.

Now you know what I'm going to say. For a given scenario I can analyse as per case A or case B ( or some other intermediate viewpoint that has both non-zero E and B ). As I please. The predictions, measured currents etc will be identical either way. I simply cannot claim either the magnet or the wire as moving or at rest in any definitive absolute sense.

This is why Einstein's paper was titled "On the electrodynamics of moving bodies". The rest is history, but the point for today is that I cannot transform both E and B away simultaneously by suitable choice of reference frame. If you think carefully about that, it must be true on other related grounds. Light is an electromagnetic wave. If I could set both fields to zero by a frame shift then I could transform light away OR if you like that is why you can't get to light's rest frame. Light has no rest frame !

[ OR you could say that the spacetime worldline of light has a proper time of zero ie. time does not pass. Hence Maxwell's equations, some of which indicate differentials with respect to time, don't resolve happily. So no regenerative wave can propagate. This is the essence of Einstein imagining 'trying to catch up to a light wave' when he was musing while taking a tram to work in Bern. ]

One final upshot is that an ensemble of charges all going this-away and that-away isn't going to have a single transform to act on the whole lot conveniently. While the Lorentz force is charge dependent, gravity acts on everything which has mass/energy. Which is everything ! Hence, as Odysseus mentions, you can convert from a 'classical frame' viewpoint where we say gravity is a force ( f = ma -->> yada yada ... ) to the GR formulation where ( differentially ) small spacetime chunks are deemed as ( sufficiently close to ) inertial frames and thus by linking such ( infinitesimally ) adjacent frames a path is generated ie. the subsequent evolution of the system is described by a geometric 'straight ruler' in spacetime ( that minimises proper time ).

Cheers, Mike.

( edit ) Aside : so in a rougher/everyday sense one can say that a moving charge generates a magnetic field, and a moving magnet generates an electric field.

( edit ) What the heck, here are the words from the very man himself ( my red highlight ) :

Quote:
ON THE ELECTRODYNAMICS OF MOVING BODIES
BY A. EINSTEIN
It is known that Maxwell’s electrodynamics—as usually understood at the present time—when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, where-as the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy, producing a current at the places where, parts of the conductor are situated. But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise—assuming equality of relative motion in the two cases discussed—to electric currents of the same path and intensity as those produced by the electric forces in the former case.
Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the “ light medium,†suggest that the phenomena of electro-dynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest. They suggest rather that, as has already been shown to the first order of small quantities, the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good.* We will raise this conjecture (the purport of which will hereafter be called the “ Principle of Relativity â€) to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell’s theory for stationary bodies. The introduction of a “ luminiferous ether †will prove to be superfluous inasmuch as the view here to be developed will not require an “ absolutely stationary space †provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.
The theory to be developed is based—like all electrodynamics—on the kinematics of the rigid body, since the assertions of any such theory have to do with the relationships between rigid bodies (systems of co-ordinates), clocks, and electromagnetic processes. Insufficient consideration of this circumstance lies at the root of the difficulties which the electrodynamics of moving bodies at present encounters.

only apparently irreconcilable will resolve/explain to mean time is not a universally agreed quantity. It becomes frame dependent and that one hits you like a wet fish in the face later on in the paper. :-) :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

merle van osdol
merle van osdol
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Thanks both of you. What can

Thanks both of you. What can I say?
I'll be working on it.
It's also here as a reference.

merle

What is freedom of expression? Without the freedom to offend, it ceases to exist.

— Salman Rushdie

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