What allows an electromegnetic wave to oscillate?

[haitch]

Oh no, I've gone and done it - I've clicked create new thread whilst in the physics/theories part of the forum. Suppose I'd better ask the question now I'm here...

What is it that causes an electromagnetic wave to oscillate? I.e. what is the medium that constrains it and pulls it back from one side to the other?

Discounting pressure waves such as sound, as they push backwards and forwards through air, other waves, with amplitude, need something to constrain them. For instance waves in water are constrained by surface tension and gravity so at the end of the "up" gravity and surface tension pulling the water flat pull it back down and vice versa at the end of the "down".

So what is doing that with visible light, gamma rays, microwaves etc? What pulls their energy back and forth causing them to oscillate?

[stonemonkeylives]

I might be mistaken but I believe its to do with the energy transference through the medium that creates the peaks and trough of waves. But I'm sure someone a lot smart than I can give you a better answer

sent from Gherkin Muncher mk .III (commonly known as a Galaxy S2)

[Cath]

Mr Maxwell is the one to ask!

But, the way I view it (in a simple way) is something like this ..

You feed an antenna with an alternating +- voltage/current, and as your doing this the energy is radiated out from the antenna in a pulse like nature (sinusoidal wave), and so the net effect is that you have a stream of sinusoidal alternating pulses, each negative peak/trough following behind each positive peak/trough. It just happens to look like a wave for an observer who is barely moving (compared to the electromagnetic energy).

So I see it as it's not really the electromagnetic energy that's oscillating on it's own, but that it's the source that's generating the alternating current.

Much better to show it with pictures rather than words, as I'm not good with words lol.

[Islander]

Yep, Maxwell's yer man.

Electromagnetic waves have two components, an electric field and a magnetic field which propagate at planes perpendicular to each other. The electric field causes the magnetic field to vary over time and the magnetic field in turn causes the electric field to vary over time. The net effect is that the EM radiation propagates in turn as a wave.

That's it in a nutshell

[Qualia]

.... or particle

[JamesF]

Do they? Or is that just the way we choose to conceptualise it?

James

[haitch]

Yep, Maxwell's yer man.

Electromagnetic waves have two components, an electric field and a magnetic field which propagate at planes perpendicular to each other. The electric field causes the magnetic field to vary over time and the magnetic field in turn causes the electric field to vary over time. The net effect is that the EM radiation propagates in turn as a wave.

That's it in a nutshell

I think you're making sense to me (which is worrying as I've polished off most of a bottle of wine whilst working on a financial model) - so instead of the boundary that the waves travel along being, say, water it is held in place by the magnetic and electric fields?

[Islander]

Do they? Or is that just the way we choose to conceptualise it?

James

They really do. Hence the ability to polarise light and radio waves.

[Islander]

I think you're making sense to me (which is worrying as I've polished off most of a bottle of wine whilst working on a financial model) - so instead of the boundary that the waves travel along being, say, water it is held in place by the magnetic and electric fields?

The electric field constrains the magnetic field and vice versa. The frequency of the wave is determined by the energy in the system.

[J_M_Franklin]

Four equations, formulated by James Clerk Maxwell , that together form a complete description of the production and interrelation of Electric and Magnetic fields. The statements of these four equations are (1) electric field diverges from electric charge , (2) there are no isolated magnetic poles, (3) electric fields are produced by changing magnetic fields, and (4) circulating magnetic fields are produced by changing electric fields and by electric currents. Maxwell based his description of electromagnetic fields on these four statements.

The first equation states that electric flux lines, if they end at all, will do so on electric charges. The second states that magnetic flux lines never terminate. The third is a form of Faraday's law of induction, which states that the rate of change of the magnetic flux threading a circuit equals the electromotive force or line integral of E around the circuit. The fourth integral is based partially on A. M. Ampère's experiments on steady currents which show that the line integral of the magnetic intensity H (or B /μ, where μ is the permeability) around a closed curve equals the current encircled.

These are the fundamental equations of classical macroscopic electrodynamics that describe electromagnetic phenomena in any medium. By using these laws as a basis and developing M. Faraday’s productive idea that the interactions between electrically charged bodies take place through an electromagnetic field, Maxwell created the theory of electromagnetic processes, which is expressed mathematically by Maxwell’s equations. The present form of the equations was given by the German physicist H. Hertz and the British physicist O. Heaviside.

Maxwell’s equations relate the quantities that characterize an electromagnetic field to its sources, that is, to the spatial distribution of electric charges and currents. In a vacuum, the electromagnetic field is characterized by two vector quantities that are dependent on spatial coordinates and on time—the electric field intensity E and magnetic induction B. These quantities determine the forces that act because of the field on the charges and currents whose distribution in space is given by the charge density ρ (the charge per unit volume) and the current density j (the charge passing in unit time through unit area perpendicular to the direction of motion of the charges). In addition to the vectors Eand B, auxiliary vector quantities that are dependent on the state and properties of the medium—the electric displacement D and the magnetic field intensity H—are introduced to describe electromagnetic processes in a material medium (matter).

Maxwell’s equations make it possible to determine the fundamental characteristics of a field (E, B, D, and H) at each point in space at any moment if the field sources j and ρ are known as functions of the coordinates and of time. The equations can be written in integral or differential form [below they are given in the absolute (Gaussian) system of units].

Maxwell’s equations in integral form determine on the basis of given charges and currents not the field vectors E, B, D and H themselves at different points in space but certain integral quantities that depend on the distribution of these field characteristics: the line integral (circulation) of the vectors E and H around any closed curve and the surface integral (flux) of the vectors D and B through any closed surface.

Maxwell’s first equation is a generalization for variable fields of the empirical law of Ampere which deals with the excitation of a magnetic field by an electric current. Maxwell advanced the hypothesis that magnetic fields are generated not only by currents flowing in conductors but also by varying electric fields in dielectrics or a vacuum. A quantity proportional to the rate of change of the electric field with time was called displacement current by Maxwell. A displacement current excites a magnetic field by the same law as does a conduction current (this was confirmed later experimentally). The total current, which is equal to the sum of the conduction current and the displacement current, is always closed.

Here is an image of how it all fits together..

A good writup on this, better than i can or have the time to do here can be found on Wikipedia, not normally know for accuracy, but this is a good article

I hope this image helps a little too.. (Credit: Oregon University Physics dept)

And whilst I am sure everyone here knows this already, i thought I would add this in for completeness. (Credit: Oregon University Physics dept)

Moving on from the above we move into Plank's Constant that details how energy at the atomic level is moved in values he called QUANTA and even today we refer to a Quanta of energy, and this is where the name QUANTUM physics comes from as Quanta is the plural of quantum.

[Islander]

Statement 2 - no isolated poles may be wrong. Quantum mechanics predicts the existence of magnetic monopoles and I remember reading an article in New Scientist outlining an experimental observation in spin ice that supported this - in fact IIRC the experimenters walked off with a major prize for the discovery.

[J_M_Franklin]

The monopole issue is there, but does not impact day to day electromagnetic theory, monopoles appear, and this may turn out to be wrong, to be limited to special environments where quantum effects begin to influence the environment.

Out mobile at the moment, doing this on my tablet, will post something more informative when I get home this evening.

[Islander]

As with most of the odd quantum effects, they're not relevant at the macro scale. However, electromagnetic theory works down to the quantum level being one of the fundamental forces. I'm not denigrating Maxwell's work - it's a profound and important observation. However, the second statement may be flawed as monopoles are predicted by quantum theory and appear to be supported by an observed real effect even if it's not a direct observation but a related effect in a condensed matter system. Dirac's statement that if magnetic monopoles are to exist then all electric charge must be quantised is supported by the simple fact that electric charge is quantised. It's not a proof but it adds weight.

The real discussion is whether monopoles are actually a part of magnetism or something else entirely.

[J_M_Franklin]

Just got in after a 15 hour day..knackered so not doing a long response now..enjoying coffee..

Monopoles are linked to magnetism, of that there is no doubt, but whether they are classical magnetism as we see in the macro world or whether they are unique to the quantum world is yet to be determined. Maxwell's theory holds water, it has shown that is experiment after experiment for the last 150 odd years, but the real question is is it the complete explanation in the same way that for everyday purposes Newtonian Gravity works perfectly well, up to a point, but as Einstein duly noted, once you move out of the everyday world you move into places where gravity has very strange effects and classical Newtonian gravitational theory falls down...we may be seeing something similar with electromagnetism.

However I have some research papers on monopoles etc so will have a reread and try to put a post together to see if we can clear this up for the level of understanding we need...can't guarantee my understanding will stretch that far though..

[George Jones]

I think that it is a little strong to say that quantum mechanics predicts magnetic monopoles. In 1931, Dirac used quantum mechanics to show that if magnetic charge exists, then the product of electric and magnetic charge is quantized. This is a very beautiful result, and highly suggestive, but it doesn't quite prove that magnetic monopoles exist.

In 1982, an experimental observation consistent with being caused by a magnetic monopole was seen. This results has never been repeated.

[Ronnie67]

Electromagnetic waves consist of a combination of oscillating electrical and magnetic fields, perpendicular to each other. This is difficult to visualize, however the waveform has similar characteristics of other types of waves.

Although they seem different, radio waves, microwaves, x-rays, and even visible light are all electromagnetic waves. They are part of the electromagnetic spectrum, and each has a different range of wavelengths, which cause they waves to affect matter differently.

The creation and detection of the wave depend much on the range of wavelengths.

When electrons move, they create a magnetic field. When electrons move back and forth or oscillate, their electric and magnetic fields change together, forming an electromagnetic wave. This oscillation can come from atoms being heated and thus moving about rapidly or from alternating current (AC) electricity.

The opposite effect occurs when an electromagnetic wave hits matter. In such a case, it could cause atoms to vibrate, creating heat, or it can cause electrons to oscillate, depending on the wavelength of the radiation.

[Islander]

I think that it is a little strong to say that quantum mechanics predicts magnetic monopoles. In 1931, Dirac used quantum mechanics to show that if magnetic charge exists, then the product of electric and magnetic charge is quantized. This is a very beautiful result, and highly suggestive, but it doesn't quite prove that magnetic monopoles exist.

In 1982, an experimental observation consistent with being caused by a magnetic monopole was seen. This results has never been repeated.

There's a world of difference between proved and predicted - I was very careful to avoid the use of proved. There's certainly evidence supporting the argument for their existence and while an absolute proof isn't there yet, the probability is that they do exist.

There was an experiment in 2009 with spin ice - specifically a supercooled crystal of dysprosium titanate in which the magnetic moments were shown to resemble dirac strings with the endpoints producing a magnetic field effect that strongly suggested magnetic monopoles. There was another paper in 2011 describing the production and measurement of monopole currents in spin ice. It won the 2012 Europhysics prize for condensed matter physics.

[badgerchap]

Sorry to be big headed, but as a Physics student, I can tell you you're all grossly mistaken.

The answer is, in fact, goats.

[Islander]

Sorry to be big headed, but as a Physics student, I can tell you you're all grossly mistaken.

The answer is, in fact, goats.

Find another Uni or a better lecturer.

It's 42.

[badgerchap]

Find another Uni or a better lecturer.

It's 42.

Sorry, I was thinking of quantum entanglement. You are correct of course, the answer is 42. 42 goats of course, but still 42.

[J_M_Franklin]

I think that it is a little strong to say that quantum mechanics predicts magnetic monopoles. In 1931, Dirac used quantum mechanics to show that if magnetic charge exists, then the product of electric and magnetic charge is quantized. This is a very beautiful result, and highly suggestive, but it doesn't quite prove that magnetic monopoles exist.

In 1982, an experimental observation consistent with being caused by a magnetic monopole was seen. This results has never been repeated.

That's not quite accurate, Paul Dirac used mathematical models to predict that at the ends of tubes then a single magnetic pole could exist, these tubes are referred to as Dirac Tubes and this is separate from the quantification of electric and magnetic charge. Dirac never proved his strings existed and they remained an elusive and much argued over part of electromagnetism for a long time.

However in 2009 researchers from Helmholtz-Zentrum Berlin für Materialien und Energie along with colleagues from St Andrews, Oxford, Dresden and La Plata published a paper in Science regarding the research they carried out at the Berlin Research reactor where they used Neutron Scattering on a single crystal of Dysprosium Titanate, they used this because it crystallizes in the so-called pyrochlore-lattice . Jonathan Morris, Alan Tennant and their colleagues have showed that the magnetic moments of the material in the lattice has reorganised itself into so called "spin spaghetti", this name comes from the ordering of the dipoles themselves, such that a network of contorted tubes, or strings, develop, through which magnetic flux is transported. These can be made visible by their interaction with the neutrons which themselves carry a magnetic moment. Thus the neutrons scatter as a reciprocal representation of the Strings. During the neutron scattering measurements a magnetic field was then applied to the crystal by the research team. Using this field they could influence the symmetry and orientation of the strings. As a result of this it was possible to reduce the density of the string networks and promote the monopole dissociation. This resulted, at temperatures from 0.6 to 2 Kelvin, in the strings becoming visible and showed they do indeed have magnetic monopoles at their ends.

The signature of the gas made up of these monopoles has also been observed in heat capacity measured by Bastian Klemke, also from Helmholtz-Zentrum Berlin für Materialien und Energie . This provided further confirmation of the existence of monopoles and showing that they interact in the same way as electric charges.

Monopoles are a hot topic now as a result of this 2009 research, as such even teams at the LHC are getting in on the act as reported on PhysOrg and there is a longer list of articles on PhysOrg HERE

If you want to read more on Monopoles there is a very good article from Cornell university on arxiv.org on the subject matter, you can find it HERE

[George Jones]

There's a world of difference between proved and predicted - I was very careful to avoid the use of proved. There's certainly evidence supporting the argument for their existence and while an absolute proof isn't there yet, the probability is that they do exist.

There was an experiment in 2009 with spin ice - specifically a supercooled crystal of dysprosium titanate in which the magnetic moments were shown to resemble dirac strings with the endpoints producing a magnetic field effect that strongly suggested magnetic monopoles. There was another paper in 2011 describing the production and measurement of monopole currents in spin ice. It won the 2012 Europhysics prize for condensed matter physics.

That's not quite accurate, Paul Dirac used mathematical models to predict that at the ends of tubes then a single magnetic pole could exist, these tubes are referred to as Dirac Tubes and this is separate from the quantification of electric and magnetic charge. Dirac never proved his strings existed and they remained an elusive and much argued over part of electromagnetism for a long time.

My post may have been incomplete with respect to the details of Dirac's monopole (beautiful mathemantics and physics), but I don't think that my post was inaccurate. Dirac's brilliant work only shows the possibility of the existence of magnetic monopoles. Dirac's work doesn't predict magnetic monopoles in the way, e.g., the standard model predicted the existence of the top quark before it was found. As one of the articles you reference below states, certain quantum field theoretical models do predict magnetic monopoles, but the status of these models currently is unclear.

However in 2009 researchers from Helmholtz-Zentrum Berlin für Materialien und Energie along with colleagues from St Andrews, Oxford, Dresden and La Plata published a paper in Science regarding the research they carried out at the Berlin Research reactor where they used Neutron Scattering on a single crystal of Dysprosium Titanate, they used this because it crystallizes in the so-called pyrochlore-lattice. Jonathan Morris, Alan Tennant and their colleagues have showed that the magnetic moments of the material in the lattice has reorganised itself into so called "spin spaghetti", this name comes from the ordering of the dipoles themselves, such that a network of contorted tubes, or strings, develop, through which magnetic flux is transported. These can be made visible by their interaction with the neutrons which themselves carry a magnetic moment. Thus the neutrons scatter as a reciprocal representation of the Strings. During the neutron scattering measurements a magnetic field was then applied to the crystal by the research team. Using this field they could influence the symmetry and orientation of the strings. As a result of this it was possible to reduce the density of the string networks and promote the monopole dissociation. This resulted, at temperatures from 0.6 to 2 Kelvin, in the strings becoming visible and showed they do indeed have magnetic monopoles at their ends.

The signature of the gas made up of these monopoles has also been observed in heat capacity measured by Bastian Klemke, also from Helmholtz-Zentrum Berlin für Materialien und Energie . This provided further confirmation of the existence of monopoles and showing that they interact in the same way as electric charges.

Monopoles are a hot topic now as a result of this 2009 research, as such even teams at the LHC are getting in on the act as reported on PhysOrg and there is a longer list of articles on PhysOrg HERE

If you want to read more on Monopoles there is a very good article from Cornell university on arxiv.org on the subject matter, you can find it HERE

Unfortunately, the interesting work on dysprosium titanate gives zero evidence for the existence of actual magnetic monopoles. I would call the features seen in this work "magnetic monopole analogues". From an article referenced above,

Last year, researchers in France and Germany reported the observation of certain states of spin ice, a kind of crystalline material with essentially the same atomic arrangements as water ice that would create monopole-like particles. But Pinfold warns, "these 'quasi-monopoles' should not be confused with the real thing being sought by the U of A led collaboration at CERN."

These aren't magnetic monopoles in the same way that electrons are electric monopoles. These magnetci monopole analogues are a collective property of matter.

To date, the only experimental evidence for magnetic monopoles of which I know is the singular 1982 event. Don't get my wrong; for decades, I have hoped that experimental evidence for magnetic monopoles would be found.

[J_M_Franklin]

George, I realise that there is a difference between the appearance of and the reality of magnetic monopoles, however I also feel that sometimes some researchers will pour scorn on others research if it is something they do not like or understand..and I think some do this without malicious intent, it is a factor of human behaviour. Clearly I do not know anymore than you whether the 2009 Research Team did actually see true Monopoles or something that appeared to be a monopole...I'll be honest when I admit that I am sitting on the fence over whether they do exist or not.

[sologuitarist61]

From Yahoo answers

Electromagnetic wave consists of oscillating electric and magnetic fields.

Electric and magnetic fields oscillate perpendicular to each other and propagate in the direction perpendicular to both.

Electric and magnetic fields can exist anywhere. They do not require any medium.

[J_M_Franklin]

OK..wanted to do this yesterday but brain was addled after a long and tiring day...and I was not thinking clearly. I have reread what i have already said on this matter and realise i have not made my position clear, so let me try again without getting overtly technical..

Alternative theories of the photon include a term that behaves like a mass, and this gives rise to the very advanced idea of the "massive photon". If the rest mass of the photon were seriously non-zero, the theory of quantum electrodynamics would, as stated by others here, be in trouble primarily through loss of gauge invariance, which would make it non-renormalisable; also, charge conservation would no longer be absolutely guaranteed, as it appears to be if photons have zero rest mass. But regardless of what any theory might predict, it is still necessary to check this prediction by doing experiments.

It is almost certainly impossible to do any experiment that would establish the photon rest mass to be exactly zero. The best we can hope to do is place limits on it. A non-zero rest mass would introduce a small damping factor in the inverse square Coulomb law of electrostatic forces. That means the electrostatic force would be weaker over very large distances.

Likewise, the behavior of static magnetic fields would be modified. An upper limit to the photon mass can be inferred through satellite measurements of planetary magnetic fields. The Charge Composition Explorer spacecraft was used to derive an upper limit of 6 × 10⁻¹⁶ eV with high certainty. This was slightly improved in 1998 by Roderic Lakes in a laboratory experiment that looked for anomalous forces on a Cavendish balance. The new limit is 7 × 10⁻¹⁷ eV. Some studies of galactic magnetic fields suggest a much better limit of less than 3 × 10⁻²⁷ eV, but there is some doubt about the validity of this last method.

At this moment in time the accepted upper value for the M_rest of the proton is 6 × 10⁻¹⁶ eV, and thus this means that to speak of the M_rest of a proton being absolute zero is not only misleading, but totally incorrect. this low level does not overtly change the world of physics as we know them, but it does mean that there is the potential for there to be variations between the M_rest of Visible light photons and the M_rest of Gamma ray photons. The real issue we have is making any measurements at this low level with current technology to any degree of precision is virtually impossible and the difference between the respective photons mass may be billionths of a percent, enough to explain the observed physical differences (energy levels) but there would be no way we will be able to even think of measuring this in the foreseeable future let alone now.

Now I know I have introduced some technical bits to the above, i should perhaps have explained myself better before, but when tired my brain gets addled and I struggle to put highly advanced concepts into everyday language, so my apologies for that failing on my part.