Re: A good new YouTube video

Paul.B.Andersen <relativity@paulba.no> wrote:

Den 16.03.2025 04:45, skrev LaurenceClarkCrossen:

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

"Where Does E = mc? 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 = mc? 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 = mc?
 
-----------------
 
This was according to NM and Maxwell. To show that E = mc? 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.

Radiation pressure, as predicted by Maxwell,
was confirmed experimentally be Lebedev in 1901.
(so well before relativity)
It is also indispensable for getting the thermodynamics
of black-body radiation right.
(also well understood, by 1901)

It did take Einstein (1905) however
to get the mass-energy equivalence right.

Jan