# Why isn't the electromagnetic field considered as the medium for electromagnetic waves by scientists in general?

According to a google search, the definition of a medium in physics is the substance that carries a wave. The definition of a substance in physics is matter with specific composition and properties. The definition of matter in classical physics is a substance (this is confusing. Is substance a type of matter, is matter a type of substance, or are the two terms interchangeable?) that has mass and takes up space.

So an electromagnetic field meets the definitions of both substance and matter. As a substance, it has a specific composition as it is made of an electromagnetic field (the most fundamental entities are made out of themselves) and it has well defined, specific properties. As matter, it produces a gravitational field and hence has gravitational mass, and also has inertial mass, as seen here: https://en.wikipedia.org/wiki/Electromagnetic_mass

It also occupies space.

Given all this, why isn't the electromagnetic field accepted as a medium for electromagnetic waves by the scientific community?

Note: the aether theory had a substance pervading all of space as an absolute frame of reference and medium through which electromagnetic waves traveled, and movement relative to it (such as earth's orbit around the sun) would cause the speed of light to change.

The special theory of relativity removed the need for an absolute frame of reference, as light (and all massless entities in general) always move at c, regardless of movement relative to them.

But I do not understand why this removed the need for the electromagnetic wave to have a medium. Special relativity removed the need for aether as an absolute frame of reference, but it did not disprove of it, and by pure logic, a wave must have a medium to propagate at all, right? So can we call the electromagnetic field an 'aether' for electromagnetic waves?

• Note that "According to a google search" is not a reference. Nov 17, 2018 at 9:28
• Then I'll just describe what terms I typed in the search bar here: "medium physics", "substance physics", "matter physics". Nov 17, 2018 at 9:33
• I disagree. As discussed in the body of the question, the electromagnetic field fulfills all the qualities of both the definitions of matter and substance. Since "medium" is the substance which carries a wave, electromagnetic fields are the medium for electromagnetic waves. The analogue would have been me asking why isn't electric and magnetic intensity the medium for electromagnetic waves Nov 17, 2018 at 12:09
• The EM field does not satisfy the definition of matter as it has no rest mass Nov 17, 2018 at 17:37
• The source of gravity/curvature is the stress energy tensor, EM fields carry energy and momentum and so can themselves induce gravitational curvature but they do not have mass and so are not matter. Nov 17, 2018 at 21:15

First - don't get too hung up on dictionary definitions of extremely broad concepts. I've been a student of physics for quite some time now, and I don't think I could give an exact definition of "matter." I know what the dictionary says, but there a lot of things which we'd call matter that don't fit that description. In fact, the dictionary, while wonderful for normal English, is a terrible resource for physics students, so I'd altogether avoid trying to reason about physics by waving definitions around.

Now, on to your question. You ask why the electromagnetic field is not accepted as a medium. The answer is that when we say that electromagnetic waves do not require a medium to propagate, what we mean is the following.

Prior to the discovery of electromagnetic waves, the waves with which we were familiar took the form of disturbances in some material, such as air, water, or rock. When we speak of the speed of such a wave, we are implicitly working in a reference frame in which the bulk material is at rest. If the medium is moving with respect to us, then we will observe a higher or lower wave speed depending on the direction of its motion.

For electromagnetic waves, this is not so. There is no such material, and there is no such reference frame.

Now, you may ask why you can't just say that the electromagnetic field is an intangible medium with no sense of rest frames, but at this point you're just playing with terminology. We specifically say that electromagnetic waves don't require a medium precisely to mean that there is no "background stuff" which carries the wave and with respect to which we are moving (or not).

Special relativity removed the need for aether as an absolute frame of reference, but it did not disprove of it, and by pure logic, a wave must have a medium to propagate at all, right?

An absolute frame of rest is incompatible with what we would now call the special theory of relativity. Also, "pure logic" is not really a good tool to use, and in this case, the answer is apparently no.

• “When we speak of the speed of such a wave, we are implicitly working in a reference frame in which the bulk material is at rest.” To be convinced about the isotropy of space according the speed of light on moving earth through space I want to see an MM experiment on the top of a television tower or the Mount Everest. Nov 17, 2018 at 12:06
• "First - don't get too hung up on dictionary definitions of extremely broad concepts."-------- my definitions were the ones used by physics. For the search terms I used, see one of the comments under the question. Nov 17, 2018 at 12:27
• @AbdulMoizQureshi It's not a question of which search terms you used. At least one neutrino species may be massless, does this make it not matter? All fundamental particles, as far as we know, are point-like and occupy zero spatial volume - does that mean that nothing is matter? The definitions you reference are extremely broad and not accurate in a technical sense. Nov 17, 2018 at 12:32
• Given that the physics definition of medium does not take rest frames into account, and I have demonstrated in the body of the question that at least by classical physics standards, the electromagnetic field fulfills the definition of a medium, then the only other possibility is that the scientific community needs to tighten its definition of "medium". Nov 17, 2018 at 12:33
• @J. Murray. By the standards of traditional classical physics, yes, that neutrino species is not matter. By modern classical physics standards, anything with mass constitutes matter, and I have demonstrated in the question body that electromagnetic fields have mass. And by the definition of "substance", EM fields have both a specific composition and specific properties. Nov 17, 2018 at 12:38

So an electromagnetic field meets the definitions of both substance and matter.

To clame the existence of a physical entity it always will be good practice to measure this entity. What one can measure is an electric field from separated subatomic particles, say an excess and a lack of electrons on the two plates of a capacitor. Furthermore a magnetic field one can measure for the aligned orientation of the magnetic dipoles of subatomic particles (see how works a permanent magnet). In both cases a test specimen of the same nature will behave as the test body. Electric fields could be measured with electrical charges, magnetic fields with magnetic dipoles.

But how about an electromagnetic field? For me an EM radiation could be set up only by the emission of photons. Ask yourself which other possibilities do you have to produce EM radiation without disturbing electrons (and other particles). Of course a stream of photons could be described over its intensity and direction and a swelling characteristic for modulated radiation. But is such a radiation a force field like an electric or a magnetic field? What are the test specimen? Photons? Of course you could measure the temperature increase of an illuminated body and the pressure on a body from the momentum of the photons. But could you draw field lines to illustrate the behavior of an EM field?

But I do not understand why this removed the need for the electromagnetic wave to have a medium.

Firstly photons are moving as a selfinductance of an electric and a magnetic field without any medium. They carry energy that the get from disturbed particles and which the give to other particles when they hit them. In between they are indivisible units on their own.

Secondly there is a spatial dependence of their behavior from the gravitational potential. Seen from a distant observer the local velocity of light slows down near Black holes and is higher far way from big masses. So a gravitational field influences the movement of light. I could claim this an aether but I want not get in trouble and so I wouldn’t.

• An electromagnetic field is made of two aspects. One is the electric field which is always present. The other is the magnetic field which manifests when the charged object creating the electric field moves with respect to an outside observer. Since both aspects can independently be measured, we say that the electromagnetic field as a whole exists. Nov 17, 2018 at 9:19
• @PM 2Rings “Seen from a far away observer ...”. See physics.stackexchange.com/questions/438921/… Nov 17, 2018 at 9:20
• You cannot equate the electromagnetic field to electromagnetic radiations. The radiation is another word for the EM wave, not the field. Other words are EM excitation, for example. As I mentioned in my last comment, their are two aspects to an electromagnetic field, and the test specimen for both the electric and magnetic aspect is the charge causing them. And for both, field lines can be drawn. Nov 17, 2018 at 9:24
• @AbdulMoizQureshi That you are right. You can express both fields in a set of equations. And this are fields from a source, the electric intrinsic charge and the intrinsic magnetic dipole of the involved subatomic particles. (This comment is related to your first comment) Nov 17, 2018 at 9:26
• "I could claim this an aether but I want not get in trouble and so I wouldn’t."------------------------------------------------------------No you wouldn't. Einstein actually had the same idea, but his understanding of the aether never gained the popularity his other theories got. Nov 17, 2018 at 9:53

Water does not move, waves move. EM field does not move, the wave moves. Field is the lowest possible state of energy. Particles are its excitation and is localized, hence particle. When you further excite the particle, that excitation move as waves through the already existing field. Empty space is not empty.There is permitivity and permeability as constant value. These constant values is what dictates the speed of propagation.

You can stick to the definition of background as some material (whatever that means) as the medium only to eventually have to face that the so called material themselves are made of that underlying field which one tossed as a medium which placed one standing on shaky ground.

In the meantime you still have to deal with particles being point like , that is it doesn't occupy spatial dimension which puts you back on the hot seat again.

When it comes to it in physics field is assumed to exist throughout empty space but not waves or forces. Fields don't move, waves do. You decide.