# Are electromagnetic waves a substance? [duplicate]

This question already has an answer here:

I would generally consider fields to not be substances, since substances are generally associated with matter. I know that energy is not a substance. Are electromagnetic waves a substance?

## marked as duplicate by ACuriousMind♦Apr 6 at 9:01

• How do you define "substance"? – Dvij Mankad Apr 5 at 21:31
• More information physics.stackexchange.com/q/364358/37364 – mmesser314 Apr 6 at 0:34
• Since electromagnetic waves are just one specific state of the electromagnetic field, this really boils down to a duplicate of whether or not fields are "substances". – ACuriousMind Apr 6 at 9:01

You won't find a definition of substance in present days university Physics textbooks. The reason is that the nineteenth century concept of substance has gone through a complete conceptual recasting after the twentieth century discoveries. Nowadays, the old word substance, as a clean scientific concept, is present only in chemistry, but with a much more limited and technical meaning.

Even the related old-fashioned distinction between matter and radiation cannot be considered anymore as a fundamental classification of what is observed and measured in laboratory.

On the basis of concepts from Relativity, Quantum Mechanics and Quantum Field Theory, a more consistent concept is that of physical system. A physical system may or may not have mass, charge, spin, it may be transformed into a different physical system. A physical system should not be confused with its properties (energy, mass, charge are properties and are not physical systems). A physical system can be considered as made of other physical subsystems, accommodating in this way the hierarchical structure of phenomenology at different time, length and energy scales.

Therefore, an EM wave is clearly a physical system.

A "substance" is generally considered to be something that has mass. Even viewing electromagnetic waves as photons (particle behavior), photons have no rest mass. So by that logic, electromagnetic waves are not "substances".

AMMENDMENT:

For the purpose of this answer, the following definitions were used

Substance: “a particular kind of matter with uniform properties"

Matter: “(in physics) that which occupies space and possesses rest mass, especially as distinct from energy”.

Hope this helps.

• But even so, electromagnetic waves carry energy and momentum. (This comment isn't for you, it's for anyone who sees your answer.) – PiKindOfGuy Apr 6 at 2:34
• Agree. That’s why I said rest mass – Bob D Apr 6 at 4:40
• Then EM (standing) wave in a box with reflecting walls would not avoid being "substance" with your logic, since it has a rest frame. – Ruslan Apr 6 at 7:23
• @Ruslan I spoke of rest mass, not a rest frame. I have amended my answer to indicate the definitions I used. Hopefully you will agree that a standing wave in a box does not meet these definitions and reconsider your down vote. – Bob D Apr 6 at 11:11
• First, it's unfair to accuse a random commenter of downvoting. I didn't vote in this thread at all. Second, your amendment doesn't actually address my comment: a pair of photons with opposite momenta does have a rest frame and thus has rest mass (it can even be confined between reflecting walls of a box). Namely, $m_{\text{pair}}=\hbar(\omega_1+\omega_2)/c$. Thus this pair could be a substance, if we follow your logic. – Ruslan Apr 6 at 13:34

A lot of Slovak and Czech physics books from Cold War era contains obligatory passage stressing how electromagnetic field is not a "substance" like water or stone, but still is a form of matter. This was often justified by "because it carriers energy and momentum". I don't think such terminology or arguments are very bright, because there are many nonunique ways to assign energy and momentum to EM field and the field is just very different from ordinary material bodies. I suspect the real reason for the insistence on this idea of field being matter was rather ideologic and even people who disagreed with this strange terminology knew better than to contradict the regime ideology.

Electromagnetic field is a very different concept from material medium (substance): it is by its nature mathematical, it obeys definite mathematical laws - Maxwell's equations. Matter is anything but precisely mathematical notion; it derives primarily from experience, you have to see and touch matter to know what "it is". Matter is very varied, it has immense number of known different manifestations (chemical compounds) and there is no general predictive universal law for its evolution as Maxwell's equations are there for EM field.

Both "matter" and electromagnetic fields are excitations of Quantum Fields. They differ in several numbers associated to them (mass, spin, etc) , but I don't see any fundamental reason to not call electromagnetic waves as matter.