What is the inertia of EM wave or field

"Inertia" is usually considered to be synonymous with mass. But mass is an official SI unit, and inertia is not. Photons are accepted to have zero mass.

John

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.

Eh?

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

age

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Their mass isn't zero - e=3Dmc^2 - though their rest mass is zero. And the speed of propagation of electromagnetic radiation varies with the medium through which the radiation is propagating, so there is some - transient - momentum transfer (think light pressure and solar sails), which makes their inertia a meaningful concept.

-- Bill Sloman, Nijmegen

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Zero *rest* mass. Remember that e=3Dmc^2 works both ways.

-- Bill Sloman, Nijmegen

Damn, I wish I knew that earlier !

Jamie

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

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

A body moving at relativistic speed effectively has a higher mass than one at rest. (There are big-endian and little-endian schools of thought on whether or not to call that a velocity dependent mass m = m_0*gamma, where m_0 is the rest mass and gamma is 1/sqrt(1-v**2/c**2).)

Cheers

Phil Hobbs

Excellent post, thank you.

Jones's work was excellent; he was clearly one of the top physicists of the era. Perhaps he gets less attention than he deserves, since he was an applied physicist. As the example at hand shows, even applied physicists can do fundamental stuff. Jones's papers on this topic make for some excellent (technical) science reading:

Jones, R. V., 1951, Nature (London) 167, 439. Jones, R. V., and J. C. S. Richards, 1954, Proc. R. Soc. London, Ser. A 221, 480. Jones, R. V., and B. Leslie, 1978, Proc. R. Soc. London, Ser. A 360,

Acoustics (with the lower wave speed) is much friendlier this way. But even worse than EM waves for space flight!

;)

After some years of looking (mostly using the automatic 'want' list on ABEBooks), I finally got a copy of Jones's "Instruments and Experiences", which reprints his papers and adds about an equal length of discussions that are very valuable for instrument builders. Highly recommended if you can find a copy.

"The Wizard War", which is about his WWII experience as the Chief of Scientific Intelligence, and his work in countering the German radionavigation beams, is also amazing. (In the UK it's called "Most Secret War".)

Cheers

Phil Hobbs

Haven't there been protests in the streets over there yet against the undemocratic nature of the royalty? You'd expect the Arab Spring/ Occupy movement (which is really all just two outcroppings of one movement) would have spread over there and would be leading to a Brain Spring to complement the Arab Spring.

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

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

One for London and one for Moscow?

Cheers

Phil Hobbs

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That's pretty cool. As the light goes through the glass the light slows down, loss of momentum. For momentum conservation the glass has to make up for the loss. He must have used some high index piece of material. I assume this is in his book. (I must have missed it on the first reading.)

George H.

This effect was first

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It'd be great if someone would reprint it.

George H.

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