What is the inertia of EM wave or field

[admformeto]

Mathew Orman

http://www.faster-than-light.us/

 

[admformeto]

The constant of the speed of light demonstrates its energy is not
kinetic. As all light waves would be of the same colour if their
energy were kinetic.

Mitchell Raemsch

There is no kinetic energy in light or EM wave as the fields are mass-less.
The question is about inertia.

Mathew Orman

http://www.faster-than-light.us/

 

[admformeto]

Photons have momentum. Look it up.

John

Yes, so they must also have inertia.

Mathew Orman

http://www.faster-than-light.us/

 

[admformeto]

There's energy associated with photons, and - as Einstein pointed out
- energy can be equated to mass. Photons may not have rest mass, but
if they were resting they wouldn't exist.


And photons have momentum and thus inertia. The question is ill-posed.

Then how much inertia a photon has?

Mathew Orman

http://www.faster-than-light.us/

 

[Chris Richardson]

They have momentum but zero mass, and only exist at one speed. I don't
think it's meaningful to talk about their inertia.

No, but we can talk about the permitivity and permeability
of free space, which are properties that determine the EM
velocity in free space, and hence can conceptually be viewed
as an "inertia" (if we stretch things).

 

[Tim Williams]

OK, what is the inertia of, say, a 500 nm photon? And what are the
engineering units of same?

Well, if one wanted to be perfectly obtuse...
https://www.google.com/search?q=planck's+constant+/+(500+nanometers+*+speed+of+light)
4.42 x 10^-36 kg

Quantum spin is in units of hbar, which is J.s = kg.m^2 / s, angular
momentum. Inertia is kg.m^2, so a quantity of 1/s (exactly, angular
frequency) is required. This would ordinarily be the rotation frequency
of the object, but a photon isn't exactly a spherical chicken, with a
well-defined shape and [angular] velocity. It's questionable whether one
could use the frequency itself here (i.e., c / lambda = 600THz); a freely
propagating photon tends to be a linearly oscillating phenomenon, but it
also does a fine job oscillating in place if confined to a resonator, so
it might not be too horrible.
https://www.google.com/search?q=planck's+constant+*+500+nm+/+speed+of+light
1.1 x 10^-48 kg.m^2

Moments of inertia of geometric shapes have the form I = a * MR^2, where
/a/ depends on the mass distribution. If we use the wavelength as the
radius and this inertia, we get a = 1, which shouldn't be surprising as:

I = a * MR^2
and
I = k * lambda / c
M = k / (lambda * c)
R = lambda

k * lambda a * k * lambda^2
------------ = ------------------
c lambda * c

a = 1, basically what we started with.

At best, this implies that all the photon's angular momentum is carried on
the periphery (a thin ring or hoop spinning on axis).

Tim

 

[Chris Richardson]

How are permittivity and permeability related to the non-inertia
of a massless photon?

The OP does not ask about photons, but about EM waves.

Since the permittivity and permeability of free space
determine the velocity of EM waves, we may want to look
upon these parameters as a kind of inertia. As I said,
it is a bit of a stretch, so don't treat this too seriously.
My purpose was to stimulate some thought on the matter
from the classical viewpoint.

 

[Gib Bogle]

"Inertia" is usually considered to be synonymous with mass. But mass
is an official SI unit

Eh?

 

[John Devereux]

Why are we feeding the troll?

He is still patiently planting links to his web site in the google
archives. Where he will shortly make available for sale his
faster-than-light cables to any gullible fool that will buy them.

(He even admits the MkI models he was selling before did not work, but
*this* time...)

 

[Jamie]

He is still patiently planting links to his web site in the google
archives. Where he will shortly make available for sale his
faster-than-light cables to any gullible fool that will buy them.

Damn, I wish I knew that earlier !

(He even admits the MkI models he was selling before did not work, but
*this* time...)

Jamie

 

[Jamie]

The OP does not ask about photons, but about EM waves.

Since the permittivity and permeability of free space
determine the velocity of EM waves, we may want to look
upon these parameters as a kind of inertia. As I said,
it is a bit of a stretch, so don't treat this too seriously.
My purpose was to stimulate some thought on the matter
from the classical viewpoint.

You were successful in one respect, you've stimulated a lot of
people but have not come away with any better results than
our country receiving stimulus packages.

Jamie

 

[PD]

What isn't meaningful is talking about photons as though they had a
separate existence, whereas they're just elementary excitations of the
EM field in a given set of boundary conditions.

EM fields have momentum and inertia--in fact the field's momentum
density is proportional to the refractive index, so if you shine a
flashlight on a glass surface, the reflected light pushes on the glass,
but the transmitted light _pulls_ on the glass. This effect was first
measured in the 1950s by R. V. Jones (one of my technical heroes) using
light bulbs and an optical lever. This is quite different from the
optical tweezers effect because it works even with plane waves, but it
isn't very big.

What makes photon drives such a losing proposition is that the
energy-to-momentum ratio of the electromagnetic field is so very big, so
it costs a ridiculous amount of power to get a tiny thrust.

(For EM fields, E=pc, whereas for matter at nonrelativistic speed, E =
p**2/(2M). The difference is a factor of 2c/v, which for a rocket with
an exhaust velocity of 4 km/s amounts to a factor of 150,000 penalty for
the electromagnetic drive versus a rocket.

As Jones says, the radiation pressure of sunlight is about equal to the
weight of one atomic layer's worth of the Earth's crust. _Not_ a big
effect.

Cheers

Phil Hobbs

Excellent post, thank you.

 

[Fred Abse]

R. V. Jones (one of my technical heroes)

Do you happen to know why he wore a watch on each wrist?

 

[Rene Tschaggelar]

A rather hefty delay ..
It is now new year and the site is still ...
lacking.

Rene