|May25-03, 02:26 PM||#1|
how does this oscillating electromagnetic field keep its information?
hi, electromagnetic radiation is a combination of oscillating electric and magnetic fields......if a transmiter sends a cmplex elctro magnetic signal into space and then shuts down, there is no longer a source of information to the signal, how does this oscillating electromagnetic feild keep its information?
|May25-03, 03:29 PM||#2|
Greetings wolram !
There are two things you should understand
to clear this up for you:
1. The electromagnetic field fluctuation
is quantized. The quant is called - photon.
It is not a continous field that is just
turned on and off. Thus if you transmit
a photon it is no longer "connected" to
the transmitter, it moves away at the speed
of light(in a vacuum) - c.
2. The EM wave - photon is indeed a wave,
but we do not know in "what". That is,
a water wave is made in water and
a more general form - a sound wave is
made in some sort of material. We do not
know what an EM wave is made in.
Possibilities may include the quantum "foam"
of space-time predicted by QM, other "hidden"
dimensions and more.
Live long and prosper.
|May25-03, 05:02 PM||#3|
Photons are not waves in anything. They have wave properties. Moreover, electrons (and other microscopic particles) do also. To quote Feynman - nobody understands quantum mechanics.
|May25-03, 05:48 PM||#4|
how does this oscillating electromagnetic field keep its information?
quite possibly Maxwell brought about a more fundamental change in how the world is viewed than did Einstein
Maxwell introduced the idea of a field extending throughout space which can oscillate independently according to M's eqns.
The field does not need hardware around (coils or charged plates or magnets) in order to oscillate. In empty space, once something (a transmitter) has started it undulating there is no reason for it to stop.
In some sense Einstein's 1905 papers are just corrections and refinements added to Maxwell's idea of a dynamic field that exists without medium. The 1905 idea of a photon is just a fixed amount of undulation in the EM field. In some way Maxwell getting the idea of the field in the first place is more of a landmark---more revolutionary---than Einstein discovering that (because of the way photoelectric tubes behave) the energy in it must be quantized.
It is never a mistake to go back and look at Maxwell's equations in free space and wonder about them-----this is what your question points to.
They are like the springbob equations where the momentum of the weight compresses the spring and the force of the spring accelerates the weight------variation in Maxwell's magnetic field sets up changes in his electric field and viceversa. Everything is reciprocal and symmetric----variation in the electric field enforces changes in the magnetic field. This happens in empty space.
It is as if the vibration of a string could happen without the string being there.
Physicists are, before anything else, believers in the reality of the field-----they behave as if fields, described by certain field-equations, actually exist. The appreciable success of physics suggests that perhaps they do exist---although it is hard to imagine how that can be.
We only see waves in water and stuff. So thinking waves in nothing is always going to be a stretch of imagination.
Remember that we evolved from fish, so dont be surprised if some things which a fish could not understand we have trouble understanding---such in any case is my view of the matter.
I will see what is the best way to write down Maxwell's equations
with PF's menu of symbols.
|May25-03, 07:58 PM||#5|
div E = 0
curl B = 1/c Et
curl E = -1/c Bt
Maxwell equations are written in many forms. this is one sophomore textbook form of the equations "in free space".
That means no charged objects around and barmagnets and stuff.
E is the electric field, a vector E1,E2,E3 defined at every point x,y,z in space and at every time t.
B is the magnetic field, a vector B1,B2,B3 defined at every point x,y,z in space and at every time t.
Et is the time-derivative of E----it is also a vector (the change is E) defined at every t,x,y,z
Bt is the time-derivative of B----likewise a vector (the change is B) defined at every t,x,y,z
Intuitively, the RHS time derivatives should be on the left because we want to know how the fields will change in terms of something about them now, in the present moment.
Maxwell says what matters in the present moment (in determining those changes) is the "curls" of E and B.
these are some partials like E2z and E3y The E partials in the curl are combined in a way that describes the extent the E field arrows go around in circles or describe eddying motion. Same for the B curl.
Partials like E1x, E2y, E3z dont occur in the curl. They describe the extent to which the field is spreading out or coming together. (As it would at point charges or point "sources".) These terms are combined in the *divergence* terms which Maxwell says are zero in free space.
div E = E1x+E2y+E3z = 0
says that in free space there are no charged particles or other sources from which the field radiates out in all directions.
There is a similar equation for divergence of B = div B.
Convergence is just the negative of divergence. Since the fields have no divergence or convergence, all they can do have the little vectors pointing around in circular eddies---which is estimated by curl E and curl B.
And, by good fortune, curl E and curl B are sufficient to determine the future changes to expect in the field
These will bring about a slightly new terrain of vectors and the new fields of vectors will have slightly new curls, and so it goes.
The curls of today's fields determine the fields of tomorrow.
And there is a lovely reciprocity-----the curl in B determines how E will change----the curl in E determines how B will change.
These are the four Maxwell equations.
Michael Faraday was a practical man who worked with coils and batteries and stuff and did hardware demonstrations and HE began to imagine fields. He thought visually and drew pictures of fields. This is where it began. Maxwell's first paper on electricity, around 1855, was called "On Faraday's Lines of Force."
Maxwell was an intellectual who put facts about fields from Faraday and others into a complete symmetric system of equations which worked together so that undulations could travel in empty space. The field became an independent dynamic entity.
The Britannica calls Maxwell's 1873 Treatise on Electricity and Magnetism "one of the most splendid monuments ever raised by the genius of a single individual."
This may in the end prove a fair assessment and not an overstatement.
|May25-03, 08:43 PM||#6|
Maxwell's Treatise on Electricity and Magnetism in Dover Paperback, or at least find a used copy. It doesn't use the "modern" div and curl notation which was actually devised by Oliver Heavyside.
|May25-03, 09:32 PM||#7|
A question for you---provide, if you can, the next four lines
that follow this:
"what immortal hand or eye,
could frame thy fearful symmetry?"
|May25-03, 10:36 PM||#8|
Eh... I have no idea of what I'm talking about now.
But [:D], I heard that Maxwell's equations are numerous (8)
and that in relativity they are all turned into
one "simple and clear" equation. So, are you experts
certain you should advise people to deal with the
enitial, supposedly quite complicated, equations ? [t)]
Live long and prosper.
|May26-03, 02:11 AM||#9|
every textbook writer seems to choose a different one
to be his favorite version there are integral versions
and so on.
The four I wrote here are verbatim from a good textbook
by Giancoli used in the 1990s for freshman and sophmore
"Physics for Scientists and Engineers,
with Modern Physics 2nd edition Prentice Hall 1989
As a real world practical matter, beginning engineering and physics majors have to get nuts and bolts Maxwell's equations firmly learned long before they have time
for more sophisitcated mathematics.
I'm not advising, I'm simply reporting how it is. Maybe someday the curriculum at colleges will be reformed and things will be done in a different order.
|May26-03, 02:35 AM||#10|
"In what distance deeps or skies
Burnt the fire of thine eyes?
On what wings dare he aspire?
What the hand dare seize the fire?"
From an excellent poem, "The Tyger," by an amazing British poet, William Blake.
|May26-03, 03:10 AM||#11|
Maxwell's equations also have a different (tensorial) form popular in general relativity -- there are only two equations in that form:
[nab][aFbc] = 0
[nab]aFab = -4[pi]jb
where Fab represents the electromagnetic field as a second rank tensor field over spacetime. j is the "current," a four vector: ([rho], j1, j2, j3). The brackets denote antisymmetry in the indices.
Also, differential forms gives us perhaps the most compact and elegant form of the equations -- the electromagnetic field is just a 2-form F (very similar to a two-index tensor, just more modern notation), with the following properties:
dF = 0
*d*F = J
where d denotes the exterior derivative, and the star is the Hodge star operator. The star operator, in general, promotes a p-form to an (n-p)-form, where n is the number of dimensions in the manifold (i.e. four in spacetime). Thus F is a 2-form, and *F is also a 2-form. The exterior derivative, in general, promotes a p-form to a (p+1)-form. Thus d*F is a 3-form, and *d*F is a 1-form.
A 1-form is, in Eucliean space, just a vector field!
J is, of course, just a 1-form on four-dimensional spacetime. If you assume that spacetime is flat (i.e. in special relativity theory), then J is just a 4-vector field defined over spacetime.
The differential form formalism is much more powerful and natural than is the tensorial representation. Many mathematicians would like to see tensors abolished entirely in favor of differential forms.
|May26-03, 01:28 PM||#12|
thanks for replies gentlemen.
my biggest problem is geting good source material, ask a local book shop if they can obtain a certain science book, at best you get a reply like (come back in two weeks we may be able to obtain a copy),
radio signals can be bounced of reflective surfaces, how does a electromagnetic "feild" bounce, or reflect ,with no loss of information ,and when close to the reflective surface how come that the "incomming " signal is not changed by interferance by the "outgoing" signal? whitch one is the leader, electric feild or magnetic feild, i mean which one gets the ball rolling?
please any good links to subject??????
|May26-03, 02:47 PM||#13|
You're most welcome ! [:)]
thing - pay for them... [:((] [;)] Use the WEB ! [:)]
The wave is absorbed by electrons and reemitted
SO energetic that they have considrable momentum in
which case they can scatter each other. EM waves can
cancel each other locally when they are equal and
have an opposite vector and occupy the same space,
but once they move "past" each other they'll be two
separate waves again.
ball rolling" is not applicable in this case. [;)]
Or, I don't understand the question.
A charged particle creates EM waves. EM waves are
relative. That is, if a charged particle is stationary
relative to you then there are no waves and all
you experience is the electric field. If it is
moving at a constant velocity there's an electric
and magnetic field and if it accelerates then EM
waves are created.
Live long and prosper.
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