Re: A good new YouTube video

Den 16.03.2025 04:45, skrev LaurenceClarkCrossen:

On Sun, 16 Mar 2025 3:36:57 +0000, LaurenceClarkCrossen wrote:
 

"Where Does E = mc² Really Come From? | The Hidden Origins of
Mass-Energy Equivalence"
 https://www.youtube.com/watch?v=xsL2nWsFgc4

The video explains how relativity assigned momentum to electromagnetism
without mass and then inferred from that that it has mass.
This is a clear case of petitio principii.

The fine art of missing the point! :-D
It isn't relativity (SR) that "assigned momentum to electromagnetism",
it was Maxwell who did that. That was the main point in the video!
E = mc² because mass-less EM-radiation has kinetic energy and momentum!
Look at the explanation of Einstein's 1906 paper in the video.
I will give a simple variant of it, using only
Newtonian mechanics and Maxwell's theory.
Given an empty box with length L and mass M.
A photon (or EM-pulse) with energy E and momentum p = E/c
is emitted from one end of the box and absorbed in the other.
When the photon is emitted, the recoil will give the box a speed
v = p/M. When the photon hits the other wall a time t = L/c later,
the box will stop. It has then moved a distance d = Lv/c, so the
centre of mass will have moved a distance d = Lv/c.
According to NM a mass m moving at the speed c will have the momentum
p = mc. So we can set Mv = mc, or v/c = m/M and d = Lm/M.
If we move a mass m the distance L relative to a mass M, the mass
centre will move a distance d = Lm/M.
Conclusion:
When a photon with momentum p = E/c move from one side of the box
to the other, this will move the centre of mass the same distance
as if a mass m with the momentum mc moved from one side of the box
to the other.
E/c = mc -> E = mc²
-----------------
This was according to NM and Maxwell. To show that E = mc² is valid
according to SR is a bit more complicated, but it is done.
My point was to show that the real reason for why mass and energy
are equivalent is that mass-less EM-radiation has energy an momentum.
--
Paul
https://paulba.no/