Why electromagnetic waves do not stand still?

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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?
 
  • #27
anorlunda
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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.
 
  • #28
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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 ??

Thanks for your input.
 
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  • #29
davenn
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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
 
  • #30
sophiecentaur
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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.
 
  • #31
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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.
 
  • #32
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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.
 
  • #33
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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'.

I (the OP) did not like the idea of antennas because they do not represent correctly the issue I am asking and additionally they also complicate things. You used the antenna and its fields to explain why the EM radiation propagate. I suggested to use just one atom in empty space to discard any arbitrary influence, like that of the antenna's fields, on the EM radiation. When just one atom emits a single packet of EM radiation into empty space, there's no other fields which influence the EM radiation, but the radiation still propagates by its own virtue. This is precisely what I want to understand. The term Photon I used was just as a matter of convenience to make it in a short form to describe emission from a single atom. In my original question I used just the EM fields and waves, no photons at all. Later I used Photons just to clarify the issue in response to your first reply. No need to get too much attached to the semantics (of photon). What's important is what is meant behind the verbal semantics. I have now already explained twice what I was meaning really in using the term Photon.
 
  • #34
anorlunda
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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.
 
  • #35
sophiecentaur
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I (the OP) did not like the idea of antennas because they do not represent correctly the issue I am asking and additionally they also complicate things. You used the antenna and its fields to explain why the EM radiation propagate. I suggested to use just one atom in empty space to discard any arbitrary influence, like that of the antenna's fields, on the EM radiation. When just one atom emits a single packet of EM radiation into empty space, there's no other fields which influence the EM radiation, but the radiation still propagates by its own virtue. This is precisely what I want to understand. The term Photon I used was just as a matter of convenience to make it in a short form to describe emission from a single atom. In my original question I used just the EM fields and waves, no photons at all. Later I used Photons just to clarify the issue in response to your first reply. No need to get too much attached to the semantics (of photon). What's important is what is meant behind the verbal semantics. I have now already explained twice what I was meaning really in using the term Photon.

I would suggest that insisting on you own interpretation and use of a term like 'photon' is not a good start for understanding an idea that is easily dealt with in classical terms. You need to realise that the example of an antenna is much easier to cope with than your 'atom' which you haven't characterised in any way. If you don't like em waves from an antenna then go for a sound wave produced in front of a loudspeaker diaphragm. Anything, in fact, that doesn't require you to involve QM. It isn't just a matter of 'symantics'. If you want rational discussion, it's only right to stick to the Physics we know and agree on.
 
  • #36
davenn
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Your answer is just turning upside down the issue without really giving any meaningful way out.

if you want to learn meaningful, then learn the physics
you are going to have to do that at a much deeper level that you seem to be prepared to do at present

The choice is yours ... simple quick answer or do a lot of study and understand the deeper answer

I suggest that maybe you should buy a copy of this and get into some study .....

41%2BM-HejEDL._SX331_BO1,204,203,200_.jpg


https://www.amazon.com/dp/1107643260/?tag=pfamazon01-20
 
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