# Why electromagnetic waves do not stand still?

Yubal M
I know that the electromagnetic waves are alternating electric and magnetic fields. But I don't get why the fields can't stand still and alternate at the same point. Why they always move ?? I would prefer a logical and verbal answer rather than a mathematical answer. But if mathematics helps to explain the logical answer then it's fine.

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Homework Helper
Waves arise from disturbances that propagate, so they move. They can appear to be 'standing' if there are two waves moving in opposite directions. That can happen to electromagnetic waves as well as to water or sound waves.

sophiecentaur
Mentor
But I don't get why the fields can't stand still and alternate at the same point.

They can. You can have standing EM waves inside a cavity with reflecting walls, just like you can have standing sound waves inside a room.

Staff Emeritus
They can. You can have standing EM waves inside a cavity with reflecting walls, just like you can have standing sound waves inside a room.

EM waves, in the form of photons, always move at the speed of light. At the beginners level, you can't ask why; it just is. At more advanced levels, you can study wave equations and Maxwells Equations.

Similarly, you can't ask why gravity makes us fall down rather than up ; it just does.

Homework Helper
What in the OP makes you think that he asked about photons?

DrStupid
EM waves, in the form of photons, always move at the speed of light.

Not always: http://extremelight.eps.hw.ac.uk/publications/Science-Express-slow-photons-Giovannini(2015).pdf [Broken]

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Staff Emeritus
Not always: http://extremelight.eps.hw.ac.uk/publications/Science-Express-slow-photons-Giovannini(2015).pdf [Broken]

Yeah, I seen similar claims. Some even claim to stop a photon completely. Frankly, I don't understand them. They seem to violate QM and Maxwells Equations. The paper cited says that it applies only to transversely structured photons, whatever that means. Other papers say that they only modified phase velocity, but this paper says both phase and group velocities. I would be grateful if someone can explain how they do this without violating the laws of physics.

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Gold Member
Photons don't provide the answer that the OP needs, I think. He seems to want a 'wave' explanation and that is in no way inferior to an explanation involving photons. Also, the same basic description applies to any kind of wave - even a Mexican Wave.
The 'movement' of a wave through space is because there is a delay in the value of the field at a point on the path of the wave, The current in an antenna changes and so does the local field but, the field thirty metres away will take 10-7 seconds to change - and so on, with increasing distance. That produces a variation in field with distance and that changes in time (along with the variations of the current in the antenna).

Homework Helper
Gold Member
... I don't get why the fields can't stand still and alternate at the same point. ...
I'm not an expert like your earlier respondants, so don't rely on my ideas, but I don't understand why you say this. At least in the context of radio waves, which is what springs to my mind when someone says "electromagnetic waves", you can.
If I stand still near an oscillating tuned circuit, then I can "see" (or detect, measure, observe on an oscilloscope, etc) an alternating electric field and an alternating magnetic field. The fields oscillate at a fixed point - indeed, at any fixed point, though their strength will be lower, the further away from the source I am.

If by some magic power I were able to travel along with the radiating radio wave at the speed of light, then I think I would not observe any oscillation in the fields at all! I would see constant phase, though monotonically diminishing amplitude..

If there is a constant current flowing in a wire near me, I expect to observe a constant magnetic field at my position. I also expect everyone else to observe the same constant magnetic field at their position - weaker if they are further away, stronger if they are nearer. Similarly if there is an electrically charged object.

If that current now varies periodically, as I first said, I expect to observe a varying field. And I expect everyone else to now observe a varying field.
So is your thought that they should not observe this varying field? That only I at some special position can observe it, but nowhere else can it be observed?

Or is your question more subtle, about why they can't observe the variations at the same time as I do,? Is the problem not that they see the same field as I do, but that they apparently see it later than I depending on how much further away they are?

So could you clarify a bit, what you mean by a field "standing still"? And perhaps whether you consider non-oscillating fields to be moving or standing still.

Although I can't really imagine the situation you speak of, if we did have an oscillating magnetic field in some fixed region and nowhere else, would that not give rise to an electric field, just as a changing magnetic field causes an emf in a wire? And since a wire loop could be anywhere where the magnetic field passes through the closed surface defined by the wire, would not that electric field have to exist, if not everywhere, at least in places going away to an indefinite distance? So even if the oscillating field could be trapped, the electric field would inevitably escape.
And I think maybe there is a similar argument for the electric field letting a magnetic field escape (though I don't understand that one, even in my naive way!).

Homework Helper
Gold Member
Adding to responses by @BvU and @jtbell ...

(with chocolate)

(with marshmallows)

(with cheese)

Neat Microwave demonstrations... (safety first!)...

Yubal M
Similarly, you can't ask why gravity makes us fall down rather than up ; it just does.

I think your claim is wrong. That's precisely the kind of question Newton is assumed to ask himself. And this question led him to formulate the Gravitation concept and theory. Asking question about things looking "obvious" is the root for real breakthroughs.

Yubal M
Thanks a lot to ALL for their replies. I find now just one reply by 'sophiecentaur' that is somewhat on the same track of my question's intention. But I need to discuss more with 'sophiecentaur' about his reply.

houlahound
Propagating waves propagate... by definition.

Standing waves stand...by definition.

Yubal M
The 'movement' of a wave through space is because there is a delay in the value of the field at a point on the path of the wave, The current in an antenna changes and so does the local field but, the field thirty metres away will take 10-7 seconds to change - and so on, with increasing distance. That produces a variation in field with distance and that changes in time (along with the variations of the current in the antenna).

Please let's leave antennas aside. Let's take just 1 atom in which an electron suddenly jumps to a lower orbiting level and so emits a photon (1 packet [quanta] of alternating electric and magnetic fields). There are no other currents or fields near this atom. So this emitted packet of alternating electric and magnetic (electromagnetic) fields is an independent packet of energy. As we already know, the electromagnetic waves (like the light) do not need any existence of any external electric or magnetic fields for moving in the Speed of Light. These waves move in empty space only by the virtue of their own alternating electromagnetic fields. So I am asking, why in the first place these alternating electromagnetic fields don't continue alternating at the same point ?? What makes these alternating electric and magnetic fields to displace their location with each cycle of their alternation ??

Gold Member
Please let's leave antennas aside. Let's take just 1 atom in which an electron suddenly jumps to a lower orbiting level and so emits a photon (1 packet [quanta] of alternating electric and magnetic fields).
The problem with jumping right into the quantum physics of a single atom transition is where you say "suddenly jumps". What do you actually mean by that? How long does the transition take and what is actually involved? You want an explanation about 'waves' so the explanation really needs to involve waves, surely.
I was using the antenna description because it describes a wave phenomenon in terms of waves. You are wanting a wave / field phenomenon described in terms of particles. That is loaded with problems. How would you actually describe the photon in terms of waves (essential if you want it to work your way)? How big is it? Where is it? We know you can't treat a photon like a little bullet - or give it an 'extent', that's meaningful.
This is a very fuzzy area in many people's minds and it will remain fuzzy unless people accept the mutually exclusive nature of the two models. It's QM and that means it will be hard / impossible to appreciate fully.
At least, consider the wave answer (based, in my case, on RF frequencies and an antenna) and see where things fit together there. I have looked all over the place for a source that links the two models together in a satisfactory way and it isn't surprising that I can't.
You ask why the fields don't keep continue alternating after the wave has 'left'. There is no energy left to sustain that oscillation as it has radiated away, being dissipated in the Radiation Resistance.

houlahound
Possibly beyond the scope of the OP question but a second quantisation approach resolves the problem ie a field mode excitation.

Gold Member
Possibly beyond the scope of the OP question but a second quantisation approach resolves the problem ie a field mode excitation.
I hoped I was answering the question at the appropriate level. The OP was asking for an answer that could be explained in terms of waves and was trying to reconcile things that are not reconcilable without digging pretty deep - if at all.

BvU
pixel
I know that the electromagnetic waves are alternating electric and magnetic fields. But I don't get why the fields can't stand still and alternate at the same point. Why they always move ?? I would prefer a logical and verbal answer rather than a mathematical answer.

In the wave, a changing electric field causes a changing magnetic field. That changing magnetic field then causes a changing electric field. Etc. Etc. So the wave propagates.

Gold Member
In the wave, a changing electric field causes a changing magnetic field. That changing magnetic field then causes a changing electric field. Etc. Etc. So the wave propagates.
But in a wave in free space, the two fields are in phase so where does that causal chain apply?

Homework Helper
What ia an "energy medium"?
Are you claiming that we don't know what a wave is and there is a need for a different definition?

Yubal M
The problem with jumping right into the quantum physics of a single atom transition is where you say "suddenly jumps". What do you actually mean by that? How long does the transition take and what is actually involved? You want an explanation about 'waves' so the explanation really needs to involve waves, surely.
I was using the antenna description because it describes a wave phenomenon in terms of waves. You are wanting a wave / field phenomenon described in terms of particles. That is loaded with problems. How would you actually describe the photon in terms of waves (essential if you want it to work your way)? How big is it? Where is it? We know you can't treat a photon like a little bullet - or give it an 'extent', that's meaningful.
This is a very fuzzy area in many people's minds and it will remain fuzzy unless people accept the mutually exclusive nature of the two models. It's QM and that means it will be hard / impossible to appreciate fully.
At least, consider the wave answer (based, in my case, on RF frequencies and an antenna) and see where things fit together there. I have looked all over the place for a source that links the two models together in a satisfactory way and it isn't surprising that I can't.
You ask why the fields don't keep continue alternating after the wave has 'left'. There is no energy left to sustain that oscillation as it has radiated away, being dissipated in the Radiation Resistance.

Thanks for trying to resolve my question. But I must say you are focusing upon various famous quantum enigmas which are not the issue of my question. It doesn't matter in my question how precisely the electromagnetic wave gets created, it doesn't matter in my question the wave-particle duality. What we know for sure, beyond any doubt, that the electromagnetic waves are alternating electric and magnetic fields in empty space (consider empty space for our simplicity). Also we know for sure that these alternating electromagnetic fields always advance and never stay at the same location (the issue of my question). It doesn't matter here their particle-type behavior in some very specific cases. I mentioned the term "Photon" just for convenience -- for to show that it's an independent and specific packet of alternating electromagnetic fields and nothing more. So there's no need to link together any models of wave and particle in my question... and so on.

Gold Member
I'm afraid you can't just wave your arms about and describe a "specific packet of alternating electromagnetic fields" and equate it to a photon. You can talk in terms of an EM pulse or a burst of a carrier wave because that's a classical entity. There is no way that you can take that directly into what happens when a single atom changes state. You cannot get away from those quantum enigmas if you want to use the word 'photon'.

Traruh Synred

EM waves, in the form of photons, always move at the speed of light. At the beginners level, you can't ask why; it just is. At more advanced levels, you can study wave equations and Maxwells Equations.

Similarly, you can't ask why gravity makes us fall down rather than up ; it just does.
You could say gravity defines up and down.

Gold Member
Propagating waves propagate... by definition.

Standing waves stand...by definition.
This needs a bit of expanding. When waves travel in both directions (back and forth and back and forth) - on a transmission line or between Etalon mirrors - the result will appear as a series of peaks and troughs of energy when a resonance is struck. But you still have moving waves and the name "standing wave" is only a word to describe what the wave pattern looks like. The Energy still goes somewhere. In fact, if there were no losses in the system, the energy in the 'cavity' (or whatever) would build up without limit. The limit to the amplitude that the pattern reaches is dependent on the amount of loss, The multiple waves (sloshing there and back, along the line) are all carrying energy which is being dissipated internally as heat or being lost by leakage to the outside. The Q of the line or cavity represents the inverse of fraction of energy lost during each cycle. It also represents a measure of how many times a wave travels back and forth before dissipating. (At least, the number of times multiplied by the number of wavelengths of the wave)

DrStupid
They seem to violate QM and Maxwells Equations. The paper cited says that it applies only to transversely structured photons, whatever that means.

The principle is well known and quite simple and it doesn't violate QM or Maxwells Equations: https://en.wikipedia.org/wiki/Bessel_beam

And in order to keep it on-topic: Such a beam is a standing wave in its own rest frame (at least for a limited period of time).

sophiecentaur
Nisse
The OP appears to be asking what it is about an electromagnetic wave that allows it to propagate through empty space under its own steam. Is that not answerable?

Yubal M
Staff Emeritus
The OP appears to be asking what it is about an electromagnetic wave that allows it to propagate through empty space under its own steam. Is that not answerable?

This thread is tagged B for beginner level. The challenge is to provide an answer at that level.

There are many physics questions that can not be answered without math and basic knowledge. In this case, the simplest answer I know it's to derive Maxwells Equations. But maybe someone more clever than I can give a beginners answer.

Yubal M
The OP appears to be asking what it is about an electromagnetic wave that allows it to propagate through empty space under its own steam. Is that not answerable?

Precisely ! You got it almost perfectly. Kudos. I think I have phrased my original question clearly and then also repeated it in different forms very clearly in my later posts. So I don't know why some complicate my question unnecessarily. I agree that perhaps the answer to my question might be very complicated, but my question is simple. I shall just correct you a little bit and say that the main point of my question is to ask, why the EM fields (of the EM radiation) don't alternate at the same point but advance at each cycle of their alteration ??

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Gold Member
The OP appears to be asking what it is about an electromagnetic wave that allows it to propagate through empty space under its own steam. Is that not answerable?

not without going into deep physics

Precisely ! You got it almost perfectly. Kudos. I think I have phrased my original question clearly and then also repeated it in different forms very clearly in my later posts. So I don't know why some complicate my question unnecessarily. I agree that perhaps the answer to my question might be very complicated, but my question is simple.

you were given the simple answer way way back ... waves, by their nature, propagate

to go any deeper than that leads to some of the other answers you were given and many of those are only scratching the surface of a very complex topic

Dave

Gold Member
The OP appears to be asking what it is about an electromagnetic wave that allows it to propagate through empty space under its own steam. Is that not answerable?
I know it's hard to read over the length of a thread before making a post but the first few posts of their thread dealt with the question quite reasonably. In post #8 I introduced the idea of delay and how that produces a wave motion. The OP didn't like the idea of introducing (classical) radio waves and their generation - because I assume he saw this as an additional problem to deal with and he/she was first in with the word 'photon'. The fact that wave motion also works the same way for mechanical and sound waves should be useful because it does not involve the dreaded photons, which are responsible for more confusion amongst 'entry level' Scientists than enlightenment.

davenn
Yubal M
not without going into deep physics

you were given the simple answer way way back ... waves, by their nature, propagate

to go any deeper than that leads to some of the other answers you were given and many of those are only scratching the surface of a very complex topic

Dave

You are defining them as waves because they propagate. I am asking why they propagate in the first place. Your answer is just turning upside down the issue without really giving any meaningful way out.

pixel
The OP appears to be asking what it is about an electromagnetic wave that allows it to propagate through empty space under its own steam. Is that not answerable?

It should be. And it appears my former response about the changing E field giving rise to a B field and vice versa is not correct. So let's try something else:

Suppose you have a charged particle oscillating up and down at some location. Let's consider when it is in the middle of its oscillation. The particle will give rise to an E field throughout space but not instantaneously - it propagates at the speed of light. So at some time later, a point, P, in space will "see" an electric field. But the particle is moving up and down so when it is, say, at the top of its motion, it will again generate an electric field moving outward at the speed of light and so at some later time the point P will experience an electric field different in magnitude from the previous one considered. So considering the continuous motion of the particle, the point P will experience an oscillating electric field. Points further out from P will also experience an oscillating field, it will just take longer for the fields to reach it.

"But I don't get why the fields can't stand still and alternate at the same point. Why they always move ??"

When you step back and look at this process, what you have is an alternating E field at each point in space with further points experiencing that at later times, i.e., they move. Etc. for the B field.

Yubal M
The OP didn't like the idea of introducing (classical) radio waves and their generation - because I assume he saw this as an additional problem to deal with and he/she was first in with the word 'photon'.

Staff Emeritus
OK, in the OP you said that you can understand some math. Try this.

First, you should understand the general wave equation. It applies not only to electromagnetic fields, but to waves at sea, sound waves, or to plucking a guitar string. Important is that all waves described by the wave equation propagate (move) at some speed. They don't stand still in space or time. (Standing waves as mentioned in post #3 are simply the sum of left moving and right moving waves.)

https://en.wikipedia.org/wiki/Wave_equation#Scalar_wave_equation_in_one_space_dimension said:
##\frac{\partial ^2u}{\partial t^2}=c^2 \frac{\partial ^2u}{\partial x^2}##
...
which leads to the general solution
##u(x,t)=F(x-ct) + G(x+ct)##

In other words, solutions of the 1D wave equation are sums of a right traveling function F and a left traveling function G. "Traveling" means that the shape of these individual arbitrary functions with respect to x stays constant, however the functions are translated left and right with time at the speed c.

Next you should understand Maxwells Equations, see Wikipedia https://en.wikipedia.org/wiki/Maxwell's_equations#Vacuum_equations.2C_electromagnetic_waves_and_speed_of_light
where you will see that solutions of Maxwells equations are a wave equation like the one above that propagates at the speed of light c.

If this answer is too difficult for the OP, and the previous answers too simple, then we have a problem in being unable to communicate. This is a case where explanation takes more a few paragraphs from a post on an Internet forum.

If you can do basic calculus, I recommend a video course https://www.youtube.com/playlist?list=PLD07B2225BB40E582 where all this stuff is explained very simply step by step. If you really yearn to know, that is admirable. You should be willing to invest some time and effort to study the topic.

davenn