A Lancaster physicist has proposed a radical solution to the question of how
a charged particle, such as an electron, responded to its own
electromagnetic field.

This question has challenged physicists for over 100 years but mathematical
physicist Dr. Jonathan Gratus has suggested an alternative
approach—published in the Journal of Physics A: Mathematical and Theoretical
with controversial implications.

It is well established that if a point charge accelerates it produces
electromagnetic radiation. This radiation has both energy and momentum,
which must come from somewhere. It is usually assumed that they come from
the energy and momentum of the charged particle, damping the motion.

The history of attempts to calculate this radiation reaction (also known as
radiation damping) date back to Lorentz in 1892. Major contributions were
then made by many well known physicists including Plank, Abraham, von Laue,
Born, Schott, Pauli, Dirac and Landau. Active research continues to this day
with many articles published every year.

The challenge is that according to Maxwell's equations, the electric field
at the actual point where the point particle is, is infinite. Hence the
force on that point particle should also be infinite.

Various methods have been used to renormalise away this infinity. This leads
to the well established Lorentz-Abraham-Dirac equation.

Unfortunately, this equation has well known pathological solutions. For
example, a particle obeying this equation may accelerate forever with no
external force or accelerate before any force is applied. There is also the
quantum version of radiation damping. Ironically, this is one of the few
phenomena where the quantum version occurs at lower energies than the
classical one.

Physicists are actively searching for this effect. This requires `colliding'
very high energy electrons and powerful laser beams, a challenge as the
biggest particle accelerators are not situated near the most powerful
lasers. However, firing lasers into plasmas will produce high energy
electron, which can then interact with the laser beam. This only requires a
powerful laser. Current results show that quantum radiation reaction does
exist.

The alternative approach is to consider many charged particles, where each
particle responds to the fields of all the other charged particles, but not
itself. This approach was hitherto dismissed, since it was assumed that this
would not conserve energy and momentum.

However, Dr. Gratus shows that this assumption is false, with the energy and
momentum of one particle's radiation coming from the external fields used to
accelerate it.

He says that "the controversial implications of this result is that there
need not be classical radiation reaction at all. We may therefore consider
the discovery of quantum radiation reaction as similar to the discovery of
Pluto, which was found following predictions based on discrepancies in the
motion of Neptune. Corrected calculations showed there were no
discrepancies. Similarly radiation reaction was predicted, found and then
shown not to be needed."

## Reference:

Jonathan Gratus, Maxwell–Lorentz without self-interactions: conservation of
energy and momentum, Journal of Physics A: Mathematical and Theoretical
(2022).
DOI: 10.1088/1751-8121/ac48ee

Tags:
Physics