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What is an electromagnetic wave?

sir drinks-a-lot

Philosopher
sir drinks-a-lot
Joined
May 19, 2004
Messages
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If I understand it correctly, it is nothing but a moving fluctuation in an electromagnetic field. But how does it move through a vaccum? What exactly is carrying the information through the vaccum?
 
The wave propels itself; kind like pulling itself by the boot straps is a common analogy.

The exact way this works can sound a little complicated, and was described in equations by James Maxwell when he was studying the relationship between magnetic force and electrical currents.

The idea in a nutshell is that an electrical current produces a magnetic field, and a magnetic field produces an electrical current. An electromagnetic wave is basically pulling itself along by alternating between a magnetic field and an electrical current. I’m sure others on here can give you a better explanation then mine however. :)
 
Dude, An elecrtro magnetic wave is what we ride when we surf the net, Duuuude...
 
ILTTL has described it fairly well. The only thing I'd add that it is a changing magnetic field that creates an electric field, and a changing electric field that creates a magnetic field. The two fields, which are at right angles, reinforce each other. It is so eloquently described by Maxwell's equations that the speed of light can be calculated from his equations.

As is so often the case in the subatomic realm, the equations describe the situation exactly while words can give us only a fuzzy "feeling" for what the equations mean....
 
<Commence Rant>

Dammit this bugs me.

Either it's photons or it's not.

Either fields and waves are explicable in terms of photons or they are not, in which case, why complicate the picture?

Spooky duality doesn't come into it. If photons are real, then waves and fields ARE photons. If so, we can dump one or the other view as redundant.

Now which is it?

</ Rant>
 
Soapy Sam said:
<Commence Rant>

Dammit this bugs me.

Either it's photons or it's not.
Click to expand...

It's photons. When you have lots of photons at once, and you measure macroscopically, you see the properties of del v cross b. That's what emerges.

But at the bottom it's all photons and QM. Even the speed of light emerges from QM.
 
sir drinks-a-lot said:
If I understand it correctly, it is nothing but a moving fluctuation in an electromagnetic field. But how does it move through a vaccum? What exactly is carrying the information through the vaccum?
Click to expand...

This question ultimately isn't any different than "what is an electric field?" Why do two charged particles at a distance from each other exert a force on each other? Why can an electric field extend through a vacuum? We can write down equations for the fields, and the force, and they work perfectly, but they don't exactly answer the question of what the field is. And to a certain extent, the question isn't answerable, because it's a fundamental thing: you can't really break an electric field into parts, and then say the field is the sum of these particular parts. You kind of just have to take it on its own terms.

But given electric fields, everything else (both magnetic fields and electromagnetic radiation) is just a natural consequence. If electric fields can extend through vacuum, then electromagnetic radiation must be able to as well.
 
Soapy Sam said:
<Commence Rant>

Dammit this bugs me.

Either it's photons or it's not.

Either fields and waves are explicable in terms of photons or they are not, in which case, why complicate the picture?

Spooky duality doesn't come into it. If photons are real, then waves and fields ARE photons. If so, we can dump one or the other view as redundant.

Now which is it?

</ Rant>
Click to expand...
Funny. :) Quantum frustration beautifully illustrated.
 
jj said:
It's photons. When you have lots of photons at once, and you measure macroscopically, you see the properties of del v cross b. That's what emerges.

But at the bottom it's all photons and QM. Even the speed of light emerges from QM.
Click to expand...

Yup. See QED (http://en.wikipedia.org/wiki/Quantum_electrodynamics).

Throw away relativity and quantum physics (or at least only consider the domains where their effects are negligible) and you get back to Classical Electromagmetics such as described by Maxwell's Equations.
 
jj said:
It's photons. When you have lots of photons at once, and you measure macroscopically, you see the properties of del v cross b. That's what emerges.

But at the bottom it's all photons and QM. Even the speed of light emerges from QM.
Click to expand...

I hadn't realized photons were charge carrying particles. I thought electrons carried the electromagnetic force.
 
ernon said:
I hadn't realized photons were charge carrying particles. I thought electrons carried the electromagnetic force.
Click to expand...
No, the electromagnetic force can cause electrons to move around, but the EM fields themselves are carried by photons.
 
I less than three logic said:
The wave propels itself; kind like pulling itself by the boot straps is a common analogy.
Click to expand...
From one perspective, nothing is being "propelled". If you look at how differential equations are studied, the problem is broken down into basic states (eigenstates), and actual states are described in terms of combinations of those basic states. From this point of view, a wave is a four dimensional entity which is not created, destroyed, or moved. It exists at all places and at all times. There are an infinite number of these waves, and whether they reinforce each other or cancel each other out depends on what time and place you're looking at, and that is what makes it look like the waves are "moving".

ernon said:
I hadn't realized photons were charge carrying particles. I thought electrons carried the electromagnetic force.
Click to expand...
Photons don't carry charge, but they do carry electromagnetic force. Think about it: if you have two electrons, what sort of interaction do they have with each other? Does charge move between them? No, they each have a constant charge. What moves between them is energy and momentum. One of them loses momentum, while the other gains momentum. One loses energy, and the other gains energy. So what is being exchanged is energy and momentum, and this energy and momentum is carried by photon.

An anology is to imagine two people on a frictionless surface throwing a baseball between each other. Suppose Person A is directly north of Person B. When Person A throws the ball, the recoil will push him north. When Person B catches the ball, the ball will push him south, and when he throws the ball again, it will push him south even faster. And so on. The result is that the two people will push each other away, just as two electrons exchanging photons will push each other away.
 
Now explain an electron and a proton attracting each other. :p

Someone should say something about virtual photons. Someone who knows more about them than I do, that is.
 
More particularly, an electric field acquires a magnetic field with strength proportional to the change in strength of the electric field, and vice versa. The two are spacial first derivatives of each other in 4 dimensions. They are observed and measured by their abilities to accellerate ordinary things.

From a more fundamental philosophical perspective, you might say "but that still doesn't say what they are", in which case you get back different answers from different philosphers. An obvious answer is they're particular patterns in the accelerations of ordinary things, and those patterns themselves exhibit changes in time that are described mathematically using modified wave mechanics, with the wave moving around 300Mm/s.

Whether such an accurate description "Describes something that is real" takes you into really basic philosophic questions of the meaning of "real" and whether, say, a rock is real.
 
Rocks are real. Trust me. I'm a geologist.

I can handle a wave / frequency description of electromagnetism and I can handle a photon / particle description.

What I do not see is where the concept of a field comes from. If a static magnet produces a static field and fields must be explained by photons, then either the photons are static, which is impossible by definition, or there must be an infinite supply of constantly renewed photons. Which sounds worryingly like something Fred Hoyle might have said. (It was always hard to know when Sir Fred was kidding).

So what is it with this "field " business? How do we get there from photons?
 
ernon said:
I hadn't realized photons were charge carrying particles. I thought electrons carried the electromagnetic force.
Click to expand...

Photons carry no charge. That much you have right. Thing is, photons propagate via V and B in free space with no charge present, too.
 
Soapy Sam said:
<Commence Rant>

Dammit this bugs me.

Either it's photons or it's not.

Either fields and waves are explicable in terms of photons or they are not, in which case, why complicate the picture?

Spooky duality doesn't come into it. If photons are real, then waves and fields ARE photons. If so, we can dump one or the other view as redundant.

Now which is it?

</ Rant>
Click to expand...

I think it is most accurate to say that light exhibits the properties of BOTH particles & waves. A photon is like the particle-version of light. I know it sounds weird, so here's an analogy...

Imagine that you're visiting a planet that has no liquids, and you are trying to explain to the natives that there is a third state of matter beyond their familiar solids & gasses. You tell them that a liquid is something that exhibits the properties of both a gas and a solid - liquids flow and fill in their containers like gasses do, yet they are also cohesive in the same manner as solids.

In this sense, light (photons) exhibit both particle & wave properties - this has been confirmed by experiment more times than I can count.

"Dumping one of these views as redundant" really wouldn't work because there are some things that can only be explained using the particle view, whereas other phenomena can only be explained using the wave view.

So we're stuck with the pesky wave-particle duality. It's not nice and neat, but who said nature was?

Cheers - Mattus
 
Soapy Sam said:
What I do not see is where the concept of a field comes from. If a static magnet produces a static field and fields must be explained by photons, then either the photons are static, which is impossible by definition, or there must be an infinite supply of constantly renewed photons.
Click to expand...
Good question - if the field around an electron is always present, no matter whether another electron is nearby to be affected by it, that implies that it must always be spitting out photons.

And I'd like to tell you that QM has a good, satisfying answer to this question, but unfortunately I can't. So physicists have come up with the concept of virtual photons, which are always present in the field, but only become real photons when there's another body there to react with. Kind of dissatisfying, ain't it?
 
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