Electromagnetic Radiation Propagation and Efficiency

In summary: But they always move. This is what makes them weird. But back to the question at hand. If you Assign a momentum to any EM radiation to explain its propagation...This is an important question. If you Assign a momentum to any EM radiation, you're implicitly saying that the radiation has a * momentum*. But that's not really what you're saying, is it? You're saying that the radiation has *properties* that correspond to momentum. And that's exactly what we've been talking about. The radiation has a momentum because that's what comes out of electromagnetic theory. But it doesn't have *that* momentum arbitrarily. It has that momentum because it is associated with a
  • #1

uby

176
0
Howdy,

I'm no physicist, but I am a scientist by trade and some basic physics questions always bug me because I don't have any bases of understanding of the concepts. I apologize if these questions seem a bit abstract:

Suppose an event occurs that causes the emission of EM radiation (for example, a transition in an electron's energy state that causes a photon to be released to satisfy conservation of energy).

1) What is the driving force for the propagation of this energy? Is it simply by conservation of momentum that it will continue to propagate even in the absence of a medium? If you, by some convention, assign a momentum to any EM radiation to explain its propagation -- then don't you also have to say that work must be done to move the radiation through any distance in space?

2) Related to the above scenario, when EM radiation propagates through space, is there any efficiency loss as the energy travels? If you assume that it performs no work on its surroundings, then I would also guess you'd have to conclude that it is perfectly efficient in that no loss of energy occurs no matter what distance the radiation travels. This seems like quite an odd thing to me!
 
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  • #2
PS - although bringing this up now is a bit premature ... the reason I ask this philosophical question is because of how I view the propagation of gravimetric radiation. that is, gravimetric radiation "propagates" not because of conservation of momentum but because of curvature of space. I would like to think, if only for reasons of symmetry and beauty, that EM radiation "propagation" could be formulated in a similar fashion -- rather than assigning momentum to the radiation, but by describing its effect on space. although I've never come across anything to suggest this to be the case.
 
  • #3
uby said:
1) What is the driving force for the propagation of this energy?

The simple answer is that no driving force is required. EM radiation simply propagates at speed c. It's difficult if you don't understand vector calculus or partial differential equations, to get into the details of electromagnetic theory. However, it suffices to say that one of the results of electromagnetic theory is that the constituent components of EM waves, namely electric and magnetic fields are governed by a "wave equation", the same "wave equation" that describes the oscillation of a medium in which there is a mechanical wave. The difference is that there is no medium. It is the fields themselves that are oscillating. In fact, a time-varying electric field produces a time-varying magnetic field which in turn produces a time-varying electric field and so on...as a result the fields are self-sustaining and can continue to propagate independently of the source that produced them. Of course, something has to get the fields propagating in the first place. This requires accelerating charge. Unmoving charge produces a *static* (not varying with time) electric field, and steadily moving charge produces a static magnetic field. As a result, there will be no fields that are changing with time, and therefore no electromagnetic wave will be radiated. If the charge is accelerating, however, then EM radiation will be produced. One way to do this is to have an oscillating electric dipole.
uby said:
Is it simply by conservation of momentum that it will continue to propagate even in the absence of a medium? If you, by some convention, assign a momentum to any EM radiation to explain its propagation...

While it is true that electromagnetic waves do carry energy and momentum, these characteristics are not arbitrarily assigned to them. (You can't do that. Something either has momentum or it does not). On the contrary, it is a fundamental *property* of EM radiation that comes out of electromagnetic theory. Conservation of momentum is always true, but it has nothing do to with whether or not EM radiation is *allowed* to propagate (again, it propagates of its own accord, as a fundamental consequence of electromagnetic theory). Similarly, the presence or absence of a medium does not affect whether light can or cannot propagate, since EM waves are not "oscillations within a medium." However, a medium does affect the speed at which EM waves propagate, since light can interact with matter.
uby said:
-- then don't you also have to say that work must be done to move the radiation through any distance in space?

First of all, EM radiation is not matter. It doesn't have mass. So talking about "doing work" to get it moving is not really correct. If we can segue awkwardly into the quantum description of light for a brief moment, the so called particles of light (known as photons) are massless. It is a consequence of the special theory of relativity that massless particles must move at speed c. *Always.* They can never be slowed or stopped (unless they are destroyed). So that's another way of looking at it. From the "photon" perspective, light must always be on the move, because its individual constituents are massless particles.

This is going to be a brief digression from the topic of light, but I wanted to point out a possible physics misconception in your above quoted statement too. Even if we WERE dealing with matter, something that has mass (such as a spacecraft ), I should point out that Newton's Second Law (the conservation of momentum you're so fond of referring to), says that an object in motion will continue in motion in a straight line at a constant velocity in the absence of any external forces. So, assuming the spacecraft was moving to begin with, then NO additional work would be required to keep it moving.

uby said:
2) Related to the above scenario, when EM radiation propagates through space, is there any efficiency loss as the energy travels? If you assume that it performs no work on its surroundings, then I would also guess you'd have to conclude that it is perfectly efficient in that no loss of energy occurs no matter what distance the radiation travels. This seems like quite an odd thing to me!

If it performs no work on its surroundings, then yes, there is no energy lost. However, in real life we don't have a perfect vacuum. There is dust etc in the interstellar medium. Light is capable of interacting with matter. Light can be absorbed by dust etc. Exactly what happens to EM waves when they encounter matter is complicated and I won't get into it here. Needless to say, the EM fields can "do work" on the atoms in the medium.
 
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  • #4
uby said:
Howdy,

I'm no physicist, but I am a scientist by trade and some basic physics questions always bug me because I don't have any bases of understanding of the concepts. I apologize if these questions seem a bit abstract:

Suppose an event occurs that causes the emission of EM radiation (for example, a transition in an electron's energy state that causes a photon to be released to satisfy conservation of energy).

1) What is the driving force for the propagation of this energy? Is it simply by conservation of momentum that it will continue to propagate even in the absence of a medium? If you, by some convention, assign a momentum to any EM radiation to explain its propagation -- then don't you also have to say that work must be done to move the radiation through any distance in space?

2) Related to the above scenario, when EM radiation propagates through space, is there any efficiency loss as the energy travels? If you assume that it performs no work on its surroundings, then I would also guess you'd have to conclude that it is perfectly efficient in that no loss of energy occurs no matter what distance the radiation travels. This seems like quite an odd thing to me!

Interesting questions, to be sure. Yes, radiation does carry energy and momentum. But, when light travels through a *lossless* medium, there is no dissipative process to decrease the energy (intensity) or change the momentum (wavevector). When losses exist, some energy is lost from the radiation as transferred to the medium (i.e. absorption) or lost in other ways (incoherent scattering).

Radiometry is the thermodynamic basis for electromagnetic waves. Radiometry makes some very odd predictions, one of which is that the spectrum of highly coherent light will change during propagation. I don't fully understand why this is so, but the results are well-documented.
 

1. What is electromagnetic radiation?

Electromagnetic radiation refers to the energy that is propagated through space in the form of electromagnetic waves. These waves are created by the movement of electrically charged particles and can travel through a vacuum.

2. How does electromagnetic radiation propagate?

Electromagnetic radiation propagates through space in a straight line at the speed of light. This means that it can travel through a vacuum and does not require a medium to travel through.

3. What is the efficiency of electromagnetic radiation propagation?

The efficiency of electromagnetic radiation propagation refers to how much of the energy of the radiation is actually transferred and received by the intended target. This can be affected by factors such as distance, interference, and absorption by other materials.

4. How is the efficiency of electromagnetic radiation propagation measured?

The efficiency of electromagnetic radiation propagation is typically measured by the amount of power that is received by the target compared to the amount of power that was transmitted. This is usually expressed as a percentage.

5. What factors affect the efficiency of electromagnetic radiation propagation?

The efficiency of electromagnetic radiation propagation can be affected by a variety of factors, such as distance, atmospheric conditions, interference from other sources, and the frequency and wavelength of the radiation. The type of material that the radiation is passing through can also impact its efficiency.

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