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rodin
2009-Jul-18, 01:01 PM
What would be the characteristics of a DC electromagnetic wave (frequency = 0)?

DrRocket
2009-Jul-18, 02:59 PM
What would be the characteristics of a DC electromagnetic wave (frequency = 0)?

There is no such thing, nor can there be.

Classically that requires a wave with no variation in time, hence there is no propagation.

Quantum mechanically it requires a photon with zero frequency, hence zero energy.

As an idealization you might be talking about a static electric or magnetic field, but that it not what most people would call a wave.

AstroRockHunter
2009-Jul-18, 07:14 PM
There is no such thing, nor can there be.

Classically that requires a wave with no variation in time, hence there is no propagation.

Quantum mechanically it requires a photon with zero frequency, hence zero energy.

As an idealization you might be talking about a static electric or magnetic field, but that it not what most people would call a wave.

DrRocket: I completely agree with you. The only thing that I can think of that would be close to a "DC electromagnetic wave" would be the short duration pulse of DC current traveling through a wire. The starting and stopping of the current would produce an EM wave of a period of 2X of the original pulse, if my wine-soaked math isn't wrong. As I recall, this is basically how spark-gap transmitters worked in the early days of Amateur Radio, the wire replaced by an air gap between electrodes.

DrRocket
2009-Jul-18, 08:46 PM
DrRocket: I completely agree with you. The only thing that I can think of that would be close to a "DC electromagnetic wave" would be the short duration pulse of DC current traveling through a wire. The starting and stopping of the current would produce an EM wave of a period of 2X of the original pulse, if my wine-soaked math isn't wrong. As I recall, this is basically how spark-gap transmitters worked in the early days of Amateur Radio, the wire replaced by an air gap between electrodes.

When you have a fast switching event like that you get a rather broad band emission. He faster the switching the broader the band.

On a rather large scale, discharging a Van de Graff generator through a network is used to simulate nuclear EMP events. You can get an EM wave like thiat, but I wouldn't call it DC.

mugaliens
2009-Jul-19, 05:32 AM
What would be the characteristics of a DC electromagnetic wave (frequency = 0)?

If you simply energized an antenna by applying to voltage potential with respect to ground, it would not be an electromagnetic wave, but merely an electric field.

rodin
2009-Jul-19, 11:01 AM
If you simply energized an antenna by applying to voltage potential with respect to ground, it would not be an electromagnetic wave, but merely an electric field.

So you can have a DC current or voltage potential, but no DC propagation through a vacuum? Only through an ionising medium as a one-way discharge?

What about an electromagnetic wave with an infinitely long wavelength?

antoniseb
2009-Jul-19, 11:10 AM
So you can have a DC current or voltage potential, but no DC propagation through a vacuum? Only through an ionising medium as a one-way discharge?

What about an electromagnetic wave with an infinitely long wavelength?

I'm not sure if you are deliberately misunderstanding, or whether you are trying to use ambiguities in the language to make a confusing argument for a novice.

You can't have a photon with an infinitely long wavelength. You can have a long lived situation in which a stream of electrons go in one direction, however, this can't be infinite in duration, because eventually you balance the charge situation.

The fact that a vacuum is involved has little to do with it (which I'm guessing you knew).

rodin
2009-Jul-19, 03:29 PM
I'm not sure if you are deliberately misunderstanding, or whether you are trying to use ambiguities in the language to make a confusing argument for a novice.

You can't have a photon with an infinitely long wavelength. You can have a long lived situation in which a stream of electrons go in one direction, however, this can't be infinite in duration, because eventually you balance the charge situation.

The fact that a vacuum is involved has little to do with it (which I'm guessing you knew).

Is there a lower limit to the wavelength of electromagnetic radiation then?

edit

Another way of putting it is what IS the lower limit to electromagnetic radiation wavelength?

rodin
2009-Jul-19, 03:35 PM
http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg

http://en.wikipedia.org/wiki/Electromagnetic_spectrum

I am interested to know what happens as we approach infinitely short and infinitely long wavelength. What are the limits and characteristics of such radiation? The 'bit in the middle' has a huge range of interactions with matter, what about the extremities?

neilzero
2009-Jul-19, 05:08 PM
Gamma photons have very short wavelengths. Obviously zero is the limit, but I don't think we can detect any shorter than about 10E-50 meters. The same thing happens at about 10E50 = there either aren't any longer and/or we don't know how to detect them. Maybe next year? Perhaps the 4 degrees k equivelent wavelength is a near zero wave length Doppler shifted to micro wave frequencies when the universe became transparent to photons about 13.7 billion years ago? Neil

antoniseb
2009-Jul-19, 07:18 PM
http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svgI am interested to know what happens as we approach infinitely short and infinitely long wavelength. What are the limits and characteristics of such radiation? The 'bit in the middle' has a huge range of interactions with matter, what about the extremities?

You've identified something that I've had a long term interest in. As to the extremities, they are both unobserved so far, AND in areas that would be testing our theories.

Recently there have been a few observations suggesting that very high energy gammas travel at less than c. Some of the String Theory and other such thoughts about the underlying structure predicted this, but as yet, we needs more observations before this is confirmed and refined. If this is true, it might be difficult for a photon to have a wavelength shorter than the size of the quantum foam.

As to the longest wavelength, I suspect that one of the longest ones in the universe is about 6 million meters, emanating from the Earth.

DrRocket
2009-Jul-19, 11:23 PM
You've identified something that I've had a long term interest in. As to the extremities, they are both unobserved so far, AND in areas that would be testing our theories.

Recently there have been a few observations suggesting that very high energy gammas travel at less than c. Some of the String Theory and other such thoughts about the underlying structure predicted this, but as yet, we needs more observations before this is confirmed and refined. If this is true, it might be difficult for a photon to have a wavelength shorter than the size of the quantum foam.

As to the longest wavelength, I suspect that one of the longest ones in the universe is about 6 million meters, emanating from the Earth.

Presumably if the energy in the universe is fixed at whatever existed with the big bang then there is an upper limit to photon frequency (E= h nu) from that consideration alone. Any practical limit would, of course, be much lower.

antoniseb
2009-Jul-19, 11:34 PM
... Any practical limit would, of course, be much lower.

Depending on the the model that String theory is hoping to be, there might well be much lower theoretical maximums in the current universe too.

Jeff Root
2009-Jul-20, 12:26 AM
This is just speculation, and maybe so obvious that I don't need to say it.
The high-energy limit is the highest-energy gamma rays that nature has
actually produced. If even higher energies than that are possible, we are
not likely to know about it. Those highest-energy gamma rays would have
been in very early moments of the Big Bang, before the Universe cooled
enough for protons and neutrons to become stable. However, all those
gamma rays are long, long gone. Even if any of them were somehow still
hanging around, they would be redshifted way, way down in the spectrum.
So the highest-energy gamma rays produced more recently would be from
hypernovae (gamma ray bursts), black holes, quasars, and in the interiors
of stars. We see all of those things either very distantly or indirectly.

The low-energy limit is the limit of what we can detect. Long radio waves
require large antennae. An antenna can detect waves longer than itself,
but I don't know how much longer. Noise is a critical limiting factor.
Maybe a supercooled, superconducting antenna and receiver in deep
Space, hundreds of kilometres in length, will detect the longest waves.
Or possibly even longer waves can be detected indirectly by their effects
on thin plasma in interstellar Space.

-- Jeff, in Minneapolis

DrRocket
2009-Jul-20, 01:14 AM
Depending on the the model that String theory is hoping to be, there might well be much lower theoretical maximums in the current universe too.

I am sure that that there is a bound lower than the very crude one that I suggested. About all that limit offers is the simple statement that if the mass-energy of the universe is finite then there must be some upper limit, i.e. that there cannot be photons of arbitrarily high frequency/energy.

As for what comes from some version of string theory, I think that depends on whether or not string theory can figure out what it wants to be when it grows up.

antoniseb
2009-Jul-20, 10:28 AM
I am sure that that there is a bound lower than the very crude one that I suggested. ....
I didn't mean it as criticism. I was just adding to your idea.

As for what comes from some version of string theory, I think that depends on whether or not string theory can figure out what it wants to be when it grows up.
Yeah, no question, regardless if it is String, or QLG, or anything else that fells that space in our model of the very small.

chornedsnorkack
2009-Jul-20, 12:30 PM
As to the longest wavelength, I suspect that one of the longest ones in the universe is about 6 million meters, emanating from the Earth.

Absurd.

Earth should be emitting photons with wavelength in the order of a million lightyears. Even as the Sun is emitting photons with wavelength of 22 lightyears. After all, any dipole that is changing its strength or direction is an electromagnetic emitter. Geomagnetic reversals should be accompanied by emission of electromagnetic waves whose frequency is, well, the frequency of geomagnetic reversals...

rodin
2009-Jul-20, 02:13 PM
This is just speculation, and maybe so obvious that I don't need to say it.
The high-energy limit is the highest-energy gamma rays that nature has
actually produced. If even higher energies than that are possible, we are
not likely to know about it. Those highest-energy gamma rays would have
been in very early moments of the Big Bang, before the Universe cooled
enough for protons and neutrons to become stable. However, all those
gamma rays are long, long gone. Even if any of them were somehow still
hanging around, they would be redshifted way, way down in the spectrum.
So the highest-energy gamma rays produced more recently would be from
hypernovae (gamma ray bursts), black holes, quasars, and in the interiors
of stars. We see all of those things either very distantly or indirectly.

The low-energy limit is the limit of what we can detect. Long radio waves
require large antennae. An antenna can detect waves longer than itself,
but I don't know how much longer. Noise is a critical limiting factor.
Maybe a supercooled, superconducting antenna and receiver in deep
Space, hundreds of kilometres in length, will detect the longest waves.
Or possibly even longer waves can be detected indirectly by their effects
on thin plasma in interstellar Space.

-- Jeff, in Minneapolis

e=hv, but what about as amplitude tends to zero?

rodin
2009-Jul-20, 02:18 PM
Absurd.

Earth should be emitting photons with wavelength in the order of a million lightyears. Even as the Sun is emitting photons with wavelength of 22 lightyears. After all, any dipole that is changing its strength or direction is an electromagnetic emitter. Geomagnetic reversals should be accompanied by emission of electromagnetic waves whose frequency is, well, the frequency of geomagnetic reversals...

You mean when solar magnetic poles flip. Do black holes or quasars flip magnetic poles?

chornedsnorkack
2009-Jul-20, 02:39 PM
You mean when solar magnetic poles flip. Do black holes or quasars flip magnetic poles?
Black holes do not have hair, including magnetic fields.

DrRocket
2009-Jul-20, 07:14 PM
e=hv, but what about as amplitude tends to zero?

That equation has nothing to do with amplitude. It describes the energy in a single quanta (photon).

The electromagnetic wave model describes the effect of a large number of photons. The amplitude is the photon density, the number of photons received per unit time. But each photon has energy described by E- h nu.

The amplitude cannot tend to zero in the usual sense of a limit in calculus. That is because the "electomagnetic wave" is actually composed of discrete photons, and there is a gap between one photon and no photons.

DrRocket
2009-Jul-20, 07:35 PM
Black holes do not have hair, including magnetic fields.

No.

They can have a magnetic field (or charge) , also mass and spin.

DrRocket
2009-Jul-20, 07:38 PM
I didn't mean it as criticism. I was just adding to your idea.

I did not take it as a criticism either.

But criticisms are OK too, since one can learn something from valid criticisms. In fact one can often learn quite a bit by being wrong and corrected.

publius
2009-Jul-20, 11:01 PM
No.

They can have a magnetic field (or charge) , also mass and spin.

This is a quite complicated mess, but basically, a black hole can have no "intrinsic" magnetic field (unless we allow magnetic monopoles, where
div B can thus be non-zero). While a rotating, charged black hole will haev a magnetic field in the stationary coordinates, that magnetic field is best thought of as an effect of frame-dragging.

It turns out a Schwarzschild black hole can have no magnetic field. Consider this, what is a magnetic field due to? Moving charge -- current. Now drop a small current loop into a Schwarzschild black hole. What happens in the external stationary coordinates? That current loop *freezes* as it approaches the horizon. The charge stops moving!

It's a complicated mess I only barely understand qualitatively, but any magnetic field that collapses with no net charge present will radiate away as EM radiation before it collapses. In models of this process, that burst can be quite powerful. Actually, in the frame of the the collapsing matter, some of the magnetic energy radiates away as radiation, escaping, while the rest is trapped and is subsumed into the singularity. That energy just becomes some of the "M", the gravitational mass of the hole to external observers.

I've seen papers about simulations of this effect of "throwing away the magnetic field as a burst of radiation". That same effect, writ much smaller will happen to a current loop dropped in a black hole.

Maxwell against curved space-time gets complex (and the coupling of Maxwell and the EFE makes even that look simple), but div B = 0 implies that magnetic field lines cannot cross an event horizon. Electric field lines since they can terminate on charge. Any net charge appears to freeze at the horizon with it's field terminating there.

With frame dragging, which manifests as space-time cross terms in the metric (time not everywhere "orthogonal" to space), stationary charge can have a magnetic field. If you're in a rotating frame with charge stationary therein, you indeed see a magnetic field! This is how a rotating, charged black hole makes a magnetic field, although the field lines do not cross the horizon.

-Richard

DrRocket
2009-Jul-21, 03:47 AM
It turns out a Schwarzschild black hole can have no magnetic field. Consider this, what is a magnetic field due to? Moving charge -- current. Now drop a small current loop into a Schwarzschild black hole. What happens in the external stationary coordinates? That current loop *freezes* as it approaches the horizon. The charge stops moving!

-Richard

Apparently that is one of the distinguishing features of a Schwarazschild black hole. But other black holes can have a field, or charge.