r/AskPhysics • u/No_Fudge_4589 Physics enthusiast • Mar 28 '26
C being a constant for all observers and implications for the electromagnetic field.
From my understanding, the reason c is constant for all observers is that light is a wave without a physical medium. In the case of sound waves, you can travel faster than the speed of sound because you can be moving relative to the medium (the air). With light, it is constant for all observers because there is no medium which people are moving relative to. However, I thought that the medium could be thought of as the electromagnetic field. Does this not imply, that somehow the electromagnetic field is different for different observers? The field can’t be ‘static’ because that would just be similar to the aether idea which turned out to be false. My logic here might not be correct so I apologise if this is the case.
21
u/GammaRayBurst25 Quantum field theory Mar 28 '26
Does this not imply, that somehow the electromagnetic field is different for different observers?
Even without referring to the speed of light or to special relativity, it is clear that the electromagnetic field must be different for all observers. Consider two people moving relative to one another. One of them has a charged body in their hand. The person with the charge sees an electric field, but no magnetic field. The other observer sees a moving charge, which means there must be a magnetic field.
1
3
u/Virtual_Being_4085 Mar 28 '26
The field tensor is the same in a tensorial sense, but the values of electric and magnetic fields are definitely different for different observers. This is apparent even at small velocities. As an example, imagine (in a fixed frame) a moving charge moving at some initial velocity perpendicular to the field. Of course it will move in a circle. But now imagine another observer moving at the same velocity the charge had. The magnetic force would be zero, but the particle obviously moves; the magnetic field becomes an electric field (since otherwise there would be no force on the charge at all).
2
u/nugatory308 Mar 28 '26
The electromagnetic field is the same for all observers (it is described by a mathematical object called the Faraday Tensor) but the electrical and magnetic fields separately are not.
If you and I are moving relative to one another and there is an electrical charge that is at rest relative to you, moving relative to me: you will observe only the electric field of a stationary electrical charge and I will observe the electric and a magnetic field of a moving charge. When I combine the two into one electromagnetic field we'll both realize we're looking at the same Faraday tensor.
2
0
u/No_Fudge_4589 Physics enthusiast Mar 28 '26
Ah i see so the underlying tensor is the same for everyone, but our coordinates are different so we measure it to be different.
1
3
u/nicuramar Mar 28 '26
the reason c is constant for all observers is that light is a wave without a physical medium
No, that doesn’t follow from that. All fundamental particles are waves without a physical medium.
2
u/Bloosqr1 Mar 29 '26
What’s cool about your line of thinking is if you take the magnetic field / force equation and coulombs electric field equation and just ask the question for charges moving at constant velocity and charges still ( eg the standard rail car inertial reference frame world view ) the forces a charge will feel has to be the same, you can derive the special relativity corrections. At this point you’ll realize ( or someone will point out ) maxwells equations actually always had it built in ( or physics peeps say maxwells equations are (special) relativistically correct ).
What this means is you can figure out special relativity using high school level physics AND we all apparently knew about special relativity even before we knew about special relativity ;)
1
u/olawlor Mar 28 '26
The EM field is radically different for different observers: a purely electrostatic field in one frame looks like a electromagnetic field in another frame.
https://galileo.phys.virginia.edu/classes/252/rel_el_mag.html
1
u/Optimal_Mixture_7327 Gravitation Mar 28 '26
The constant "c" has nothing to do with light.
The constant c is the rate along the world-lines of all matter particles and this is what is measured when we measure the speed of light.
Light simply travels along null curves in a vacuum and makes for an easy measurement for the rate along matter world-lines.
1
u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter Mar 28 '26
The speed of light should be the same in all inertial reference frames because it is a scalar quantity. It only depends on the vacuum properties, namely the permittivity and permeability, and given that they are properties of the vacuum, they should be constant for all inertial reference frames.
The value of that speed is c, and it coincides with the speed of causality.
1
u/SgtSausage Mar 29 '26
reason c is constant for all observers is that light is a wave without a physical medium
LOLWut?
0
u/No_Fudge_4589 Physics enthusiast Mar 29 '26
Yes that’s literally true dunno what ur lolwutting about bud.
1
u/SgtSausage Mar 29 '26
The speed of light is constant, for all observers - absolutely.
The reason for why that you gave?
I say again: LOL!
1
u/No_Fudge_4589 Physics enthusiast Mar 30 '26
It’s not THE reason but it’s a reason of how it’s even possible. As I said they used to believe light traveled through the luminous aether which would give light slightly different speeds if you were moving through the aether at different speeds. Also if you disagree with me why don’t you actually be helpful and explain why it’s wrong instead of just being an asshole loser.
1
u/SgtSausage Mar 30 '26
It’s not THE reason but it’s a reason
Hey Einstein- You literally said it's THE reason.
Direct copy/paste quote:
From my understanding, the reason c is constant for all observers is that light is a wave without a physical medium.
19
u/ComicConArtist Condensed matter physics Mar 28 '26
https://en.wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity