This paper is in the public domain and people have uploaded it e.g. to glorious Wikisource: en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether including its amazing illustrations.

What you would see the moving rod look like on a photo of a length contraction experiment, as opposed as using two locally measured separate spacetime events to measure its length.

This is a well known though experiment, which Richard Feynman used to emphasize

In the above experiment:

The solution to this problem is length contraction: the positive charges are length contracted and the moving electrons aren't, and therefore they are denser and therefore there is an effective charge from that frame.

This is also mentioned at David Tong www.damtp.cam.ac.uk/user/tong/em/el4.pdf (archive) "David Tong: Lectures on Electromagnetism - 5. Electromagnetism and Relativity" "5.2.1 Magnetism and Relativity".

A 4D gradient with some small special relativity specifics added in (the light of speed and sign change for the time).

This form is not really an inner product in the common modern definition, because it is not positive definite, only a symmetric bilinear form.

TODO arguments, proofs

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The opposite of quasiparticle, see notaby: quasiparticles vs elementary particles.

Initially light was though of as a wave because it experienced interference as shown by experiments such as:

But then, some key experiments also start suggesting that light is made up of discrete packets:and in the understanding of the 2020 Standard Model the photon is one of the elementary particles.

This duality is fully described mathematically by quantum electrodynamics, where the photon is modelled as a quantized excitation of the photon field.

The history of light if funny.

First people thought it was a particle, as per corpuscular theory of light, notably Newton supported the corpuscular theory of light.

But then evidence of the diffraction of light start to become unbearably strong, culminating in the Arago spot.

And finally it was understood from Maxwell's equations that light is a form of electromagnetic radiation, as its speed was perfectly predicted by the theory.

But then evidence of particle nature started to surface once again with the photoelectric effect. Physicists must have been driven mad by all these changes.

The book is a bit slow until Charles K. Kao comes along, then it gets exciting.