|
Post by Progenitor A on Jan 18, 2011 13:35:30 GMT 1
But on the side of the wheel that did experience gravity, gravity would act uniformly to the front and rear, Why would it rotate? Imagine you are looking along the (horizontal) axis of the wheel, and the part of the wheel to the right of the axis is shielded from gravity. The left side of the wheel is being pulled by gravity, so it will rotate anti-clockwise. Ah yes! I see
|
|
|
Post by speakertoanimals on Jan 18, 2011 14:41:13 GMT 1
Naymissus trying to spread confusion as before!
I've been through all this before, the point being a field can be described (ie you know all information about it) either by giving the value of the potential at every point, OR by giving the value of the field strength (gradient of the potential) at every point.
But the value of the potential AT A SINGLE POINT is NOT the same as giving the field strength at that point (which may be any value, if all you have to go on is the value of the potential at a single point).
All comes down to basic calculus, and the fact that the field strength (i.e., physical forces) don't depend on the absolute value of the potential.
Why certain characters want to keep playing silly word games and claiming that potential and gradient of potential mean the same thing is beyond me, unless all they are on about (no matter what disinformation they spread), is trying to convince themselves they hqve made me look silly (whereas all that is ever true is that they make themselves look even sillier than usual, if that is possible).
More silly word games! A field can exist, yet have a zero value at some set of points, or over some region. For the field to NOT EXIST, it would have to be zero everywhere. Hence all we have to do to stop daft sounding statements is be a bit more PRECISE about what we mean by exist.
So, a field exists, even if it happens to have zero value at some place at some moment, but it may not have zero value at that place at some later moment. Your problem is in taking 'has zero value' (which is prefectly MEASURABLE, just stick a test particle there!) to be synonymous with 'does not exist'.
So, to play about with 'does not exist' magnetic fields exist, whether or not you happen to have a region with for the moment is free of them (so, to be more precise, magnetic fields don't exist AT THAT PLACE at that instant, but do exist in the wider sense). which is a different sort of statement to saying that magnetic monopoles don't exist, since we believe that they really don't exist, not now or at any other time or place.
|
|
|
Post by Progenitor A on Jan 18, 2011 15:19:43 GMT 1
. In a small cave at the center of the earth, does the gravitational field exist? If so, how do we measure it ? Yes it does exist but its value is zero. Sounds daftish but true The only way of verifying that there is a (zero value) gravity field there is to somehow shield a portion of the cave from gravity (I am working on how to do thatthat) and out would pop a gravitational field in the rest of the cave - out of nothing apparently! A (simple case of): Nothing - something = something (arithmetically sound of course) related to Something + something = nothing (also arithmetically sound) So the presence of nothing does not (necessarily) lead to the inference that there is nothing there! Which is your point of course!
|
|
|
Post by speakertoanimals on Jan 18, 2011 15:23:35 GMT 1
Wrong. The presence of a zero-value gravity field is just shown by placing a test object there, and observing that it does not move.
The existence of a zero-value gravity field doesn't need to be shown by showing that a non-zero gravitational field exists somewhere else, we already KNOW that non-zero value gravity fields exist!
If you could work out how to shield part of the cave from gravity, you'd get a Nobel prize, so stop being daft -- any argument that relies on such a shield can be dismissed as worthless without going any further!
|
|
|
Post by carnyx on Jan 18, 2011 15:27:48 GMT 1
STA
Instead of ranting and raving, and spewing word-salad to such an extent that you contradict yourself, I should like you to stick to commenting on the idea of a force-field ( electric, magnetic, or gravitational) that exists yet cannot be detected.
It seems that, as such fields are detectable only if the force has a direction i.e. it is a vector quantity. But say in the equidistand points between positively charged plates or the opposed north-poles of magnets; there is no direction; hence no resultant vector .... It really seems that such 'flat' field areas can exist undetected.
However, a local disturbance in this flat field area will produce local gradients, which can be detected. And we could say that such disturbances will propagate across this field.
So it may well be that EM radiation is actually a propagating sympathetic disturbance in the universal electric, and magnetic, force-fields, and not in 'nothing' .....
It follows that space is not 'empty' at all.
Then, I'd like to ask you about the analogous case of a gravity field. In a small cave at the center of the earth, does the gravitational field exist? If so, how do we measure it ? And in this cave, will we still be able to measure the gravitational force of the sun, and the moon?
|
|
|
Post by speakertoanimals on Jan 18, 2011 15:46:05 GMT 1
nonsense. A 'flat' field area CAN of course be detected, by just observing that a charged particle experiences NO force. Detected, easy-peasy.
A null vector is still a vector, and a value of zero is STILL a value. If we perturb, then we may get non-zero values, but rather than the discontinuous process (magically from no field to some field), we instead have field of zero value within some region, or at some point, being perturbed to having a non-zero value.
Except we already know that electric and magnetic fields exist in our universe, whether or not they have non-zero value in some particular region. The potential (hah-hah, I can play word games too) for fields to exist in some region doesn't get round the what are the em field carried by non-question.
I've already answered this. It is no more mysterious than the fact that in a complicated system of gravitating bodies, there will be points where the forces cancel to give no net gravitational force at various points. The case of a hollow sphere (and nothing case), is just a nice case where the cancelllation works over a volume rather than at a single point.
Do stop talking nonsense, and stop claimintg contradictions where none eist, or that all sentence constructions other than the totally simple are word-salad...........
|
|
|
Post by carnyx on Jan 18, 2011 18:38:44 GMT 1
STA
You say;
"we already know that electric and magnetic fields exist in our universe, whether or not they have non-zero value in some particular region. The potential ..........for fields to exist in some region doesn't get round the what are the em field carried by non-question."
Thank you for going some way towards the idea that;
1. The universe has continuous electric, magnetic, and gravitational fields.
2. Local disturbances in the electric and magnetic fields propagate EM waves in their respective fields.
Now with a single continuous magnetic field, we can look at, say, a magnet and say that it is producing a kind of 'dimple' in the magnetic field of the universe. In other words the magnetism does not emanate from the magnet, but from a local distortion of the universe's all-pervasive magnetic field.
( But, I note you haven't answered this significant question;
"... in this cave, will we still be able to measure the gravitational force of the sun, and the moon?)
|
|
|
Post by speakertoanimals on Jan 18, 2011 21:29:50 GMT 1
WHEN did I ever disagree with 1 (although I could quibble over the use of continuous in a mathematical sense, since for problems we often have boundary conditions that introduce discontinuities, like when we have a definite boundary corresponding to a change of material).
I will disagree with 2), since you seem to be implying disturbances in the fields propogate waves. This is wrong!
Disturbances in the SOURCE of the fields (moving charges, currents, magnets or masses) induce changes in the fields. These changes propogate outwards from the source, but that isn't quite the same as em waves as usually understood.
It is, if you like, the difference between a messy step-like pulse propogating outwards, which is the change in the field between two states, A and B, and a sustained wave-like excitation propagating outwards.
Nope. Because we can place the magnet in a region where there is no magnetic field (or to be precise, a region where the magnetic field is zero), and the magnetic field is zero plus the magnetic field created by the magnet.
So, magnetic fields ADD, provided the source of one doesn't effect the source of another.
But since they add, you can't use the fact that there may or may not have been a magnetic field before you started, to explain what 'carries' the second magnetic field that appears when youi switch the electromagnet on. There is nothing (apart from zero everywhere), to act as a universal basis for your fields.
So, in your case, the 'magnetic field of the universe' could be replaced by the quantum vacuum state of the quantised em field -- which is, in effect, what quantum field theory does. It is then lowest possible energy state of em fields.
SO, in that sense, it can be thought of as a little like an 'aether', BUT only by introducing specifically quantum ideas, and it shares very little with conventional classical ideas of a medium for propogating em waves. It doesn't actually answer the supposed classical objection to what do em waves propagate through, but it does answer more complicated questions as to the quantum nature of the em field. But since some on here still don't believe in photons..........
I did actually. You ONLY get zero gravitational fiorce across a hollow sphere if there are no other gravitating bodies about (like the sun or moon), and only if the mass distribution of the sphere is perfectly symmetrical. No real body is totally symmetric, so the non-zero regions inside a real sphere are just the same sort of perturbations as caused by the presence of other gravitating bodies outside the hollow sphere.
Just in case SOME people got the view that a hollow sphere somehow magically screened out the gravitational effects of bodies outside the sphere to create the zero field region................
I might just add that you won't get significant gravitational effects from the sun and moon, not because they aren't there, but because we are free-falling in response to the fields of the sun and the moon. What you would detect is the tidal effects of the sun and the moon. But the fact that the sun pulls on you is proven by the fact that you will continue to orbit the sun, along with the earth. Relative to the earth, you will feel a slight additional pull, when you're not exactly at the centre of mass of the earth, and that is the tidal effect due to the sun. # Ditto for the moon, but now we are talking about 3 mutually-orbiting bodies, and it gets a little more complicated!
|
|